My Mac is asking me for a system recovery pin. I have never seen this before

That usually means the Mac was marked as lost or stolen, or somebody has your Apple ID and password and marked the Mac as lost or,stolen, and has set a passcode lock.

If your Apple ID is the ID holding the passcode lock and you don’t recall or don’t know the passcode, then it can usually be unlocked from the website:

https://iCloud.com/find

selecting the Mac from the devices list, and choosing unlock. Otherwise, you will need to work with Apple or an authorized Apple repair provider to clear the lock.

If your Mac is lost or stolen - Apple Support

If you're the one and only owner of the Mac, and you have not engaged iCloud lock, you should have your Apple ID credentials checked

If you think your Apple ID has been compromised – Apple Support (UK)

Are you using macOS, or Windows? (Windows has a recovery PIN, but macOS does not.)

There is a system lock PIN, which can be a factor if this Mac was not transferred from a previous owner.

Post a picture of the prompt?