This volume is not formatted as APFS

If you are seeing this screen with a padlock

then you have an EFI firmware password set which can be different than your user account password. If you don't recall the password, then you will need to provide valid proof of purchase to Apple to that Apple can remove the firmware lock. See this Apple article for more details:

https://support.apple.com/en-us/HT204455

If this is not the issue, then try booting into Internet Recovery Mode using Command + Option + R. Otherwise we will need more information on exactly where you are seeing the need to unlock the system. Maybe provide a picture.

You need to wipe the drive and reinstall Mojave. You'll want to back up your info prior to this. Reinstalling Mojave after wiping it will format the hard drive to the correct format. Afterwards, you can install Catalina and then put your info back.