I forgot my find my Mac lock code when it was in lost mode

Refer to the following passage from Mark a device as lost in Find My on Mac - Apple Support:

Unlock a Mac

When you find your lost Mac, enter the numeric passcode on the device to unlock it (the one you set up when you marked your Mac as lost).

Since you do not know that code, the next sentence applies to your concern:

If you forget your device passcode, you can recover it using Find My iPhone on iCloud.com. See Use Lost Mode in Find My iPhone in the iCloud User Guide.

shaney209 Said:

"How can I get into my Mac if I forgot the lock code?"

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Use the Login Screen:

Reset it using the login screen. You'd go through your Apple ID to perform this. Find out more here: If you Forgot your Mac Login Password - Apple Support