No, you are not source of the problem! Apple messed you up!
Apple lets its OS installer certificates expire to force people to newer OS’s and/or new hardware! Your system can support upto macOS High Sierra (10.13.x). But even if you did apply the '''Option-⌘-R''' keys during the boot-up you will still be out of luck! As Apple messed up by not maintain the certificate keys even on these older images too! [https://appleinsider.com/articles/19/10/24/if-youve-got-an-old-macos-install-image-it-will-probably-stop-working-today|If you've got an old macOS install image, it will probably stop working today] Here’s the needed OS image which has the new certificate [https://support.apple.com/en-us/HT208969|How to upgrade to macOS High Sierra] Jump to Step 4 to get the file link.
OK, So now I have the needed file how do I use it? Follow this guide to create a bootable OS installer USB Thumb drive [https://www.macworld.com/article/3204672/how-to-create-a-bootable-macos-high-sierra-installer-drive.html|How to create a bootable macOS High Sierra installer drive] from a second Mac. Then use it to boot up your system to then install a fresh OS.
'''References:'''
* [https://support.apple.com/en-us/HT201255|Mac startup key combinations]
* [https://support.apple.com/en-us/HT204904|How to reinstall macOS from macOS Recovery]