I think you’ve fallen into the crack! Halfway between HFS+ and APFS!
Apple updated its file system from HFS+ to APFS and from the sounds of it your original system was using High Sierra (10.13.x) or upto Big Sur (macOS 11.x)! But the logic board you got likely still had the older firmware which doesn’t know how to boot up under APFS and of course! Mountain Lion has no clue about it either so it can’t access the drive and won’t let you install as there is no room as the drive already has the partition laid out as APFS!
'''OK, how you get out of this mess?''' You’ll need to use another Mac to create a bootable USB thumb drive OS installer. We need one that is able to update your systems firmware to APFS support. So I would use Mojave. First use your other Mac to format the 32GB USB thumb drive with GUID and a journaled file system. Then download Mojave from here [link|https://support.apple.com/en-us/HT211683|How to get old versions of macOS] and follow this guide to convert it to a bootable image on the USB thumb drive [link|https://www.macworld.com/article/3284378/how-to-create-a-bootable-macos-mojave-installer-drive.html|How to create a bootable macOS Mojave installer drive]
Then using it you’ll boot up under it but we have one small wrinkle! We don’t want to damage your current OS on your internal drive as such we need a second external drive (USB or Thunderbolt) to install the OS and during that process the firmware will be updated.
OK, now we have a bootable USB thumb drive with Mojave and you should now be able to see your internal drive, booting up under the temp drive If you do you should be able to figure out what your systems OS level is, unless you can remember. Then upgrade this USB temp drive to that OS at which point you only need to then alter your startup using the Option key to gain access to the Startup Manager and then set it permanently via your Startup Disk control panel.
I think you’ve fallen into the crack! Halfway between HFS+ and APFS!
Apple updated its file system from HFS+ to APFS and from the sounds of it your original system was using High Sierra (10.13.x) or upto Big Sur (macOS 11.x)! But the logic board you got likely still had the older firmware which doesn’t know how to boot up under APFS and of course! Mountain Lion has no clue about it either so it can’t access the drive and won’t let you install as there is no room as the drive already has the partition laid out as APFS!
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'''OK, how you get out of this mess?''' You’ll need to use another Mac to create a bootable USB thumb drive OS installer. We need one that is able to update your systems firmware to APFS support. So I would use Mojave. First use your other Mac to format the 32GB USB thumb drive with GUID and a journaled file system. Then download Mojave from here [https://support.apple.com/en-us/HT211683|How to get old versions of macOS] and follow this guide to convert it to a bootable image on the USB thumb drive [https://www.macworld.com/article/3284378/how-to-create-a-bootable-macos-mojave-installer-drive.html|How to create a bootable macOS Mojave installer drive]
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'''OK, how you get out of this mess?''' You’ll need to use another Mac to create a bootable USB thumb drive OS installer. We need one that is able to update your systems firmware to APFS support. So I would use Mojave. First use your other Mac to format the 32GB USB thumb drive with GUID and a journaled file system. Then download Mojave from here [link|https://support.apple.com/en-us/HT211683|How to get old versions of macOS] and follow this guide to convert it to a bootable image on the USB thumb drive [link|https://www.macworld.com/article/3284378/how-to-create-a-bootable-macos-mojave-installer-drive.html|How to create a bootable macOS Mojave installer drive]
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Then using it you’ll boot up under it but we have one small wrinkle! We don’t want to =damage your current OS as such we need a second external drive (USB or Thunderbolt) to install the OS and during that process the firmware will be updated.
+
Then using it you’ll boot up under it but we have one small wrinkle! We don’t want to damage your current OS on your internal drive as such we need a second external drive (USB or Thunderbolt) to install the OS and during that process the firmware will be updated.
OK, now we have a bootable USB thumb drive with Mojave and you should now be able to see your internal drive, booting up under the temp drive If you do you should be able to figure out what your systems OS level is, unless you can remember. Then upgrade this USB temp drive to that OS at which point you only need to then alter your startup using the Option key to gain access to the Startup Manager and then set it permanently via your Startup Disk control panel.
I think you’ve fallen into the crack! Halfway between HFS+ and APFS!
Apple updated its file system from HFS+ to APFS and from the sounds of it your original system was using High Sierra (10.13.x) or upto Big Sur (macOS 11.x)! But the logic board you got likely still had the older firmware which doesn’t know how to boot up under APFS and of course! Mountain Lion has no clue about it either so it can’t access the drive and won’t let you install as there is no room as the drive already has the partition laid out as APFS!
'''OK, how you get out of this mess?''' You’ll need to use another Mac to create a bootable USB thumb drive OS installer. We need one that is able to update your systems firmware to APFS support. So I would use Mojave. First use your other Mac to format the 32GB USB thumb drive with GUID and a journaled file system. Then download Mojave from here [https://support.apple.com/en-us/HT211683|How to get old versions of macOS] and follow this guide to convert it to a bootable image on the USB thumb drive [https://www.macworld.com/article/3284378/how-to-create-a-bootable-macos-mojave-installer-drive.html|How to create a bootable macOS Mojave installer drive]
Then using it you’ll boot up under it but we have one small wrinkle! We don’t want to =damage your current OS as such we need a second external drive (USB or Thunderbolt) to install the OS and during that process the firmware will be updated.
OK, now we have a bootable USB thumb drive with Mojave and you should now be able to see your internal drive, booting up under the temp drive If you do you should be able to figure out what your systems OS level is, unless you can remember. Then upgrade this USB temp drive to that OS at which point you only need to then alter your startup using the Option key to gain access to the Startup Manager and then set it permanently via your Startup Disk control panel.
Reference: [https://support.apple.com/en-us/HT201255|Mac startup key combinations]