Monterey won’t install on third-party ssd.

because of the EFI Boot ROM firmware requirement, you cannot upgrade to monterey with a 3rd party SSD. AFAICT the only option is to reinstall an original Apple drive, upgrade that to monterey, then reinstall the 3rd party SSD and then you can upgrade it to monterey. if you have no access to an original Apple SSD, (if you can't beg or borrow) you are going to have to put off the upgrade to monterey unless / until apple changes this requirement. if it was me, i would contact OWC and see if they have any second hand apple SSDs in stock. they do apple hardware upgrades, so maybe...

If you install Mojave 10.14.6 using the Apple SSD, the new version firmware 130.0.0.0 is said to be freely upgradeable after that, without the use off the Apple SSD in place.

If you no longer have a working Apple SSD, you could buy one on eBay to use for the upgrade.