BootCamp can only run from the boot drive. This is an Apple limitation.
Even still you appear to be having a more basic issue, from your description it sounds like you are using a M.2 SSD with an adapter. These can be tricky! More so when used in an iMac as it’s a bear to go back in maybe two or three times trying to get it to work. Frankly, they are more of a pain than worth it.
I would stick with either a real Apple SSD or use the OWC SSD which is a true pin compatible drive. Yes, both are more expensive you can thank Apple for not sticking to an industry standard.
I would also run Apples Disk Utility from your HDD until you can see the drive within it. If you can’t the connection and/or the adapter and SSD are bad.
BootCamp can only run from the boot drive. This is an Apple limitation.
Even still you appear to be having a more basic issue, from your description it sounds like you are using a M.2 SSD with an adapter. These can be tricky! More so when used in an iMac as it’s a bear to go back in maybe two or three times trying to get it to work. Frankly, they are more of a pain than worth it.
I would stick with either a real Apple SSD or use the OWC SSD which is a true pin compatible drive. Yes, both are more expensive you can thank Apple for not sticking to an industry standard.
* [https://beetstech.com/product/a1418-2013-solid-state-drive|Apple 1TB SSD]
* [product|IF123-133-3]