You will almost certainly need to contact Apple Support, but just so that I understand: once you get to that screen and choose a User Account, is there no option to provide a password, or an Apple ID for that user?
The several options that follow that step begin with "if" because not all of them may be available for all User Accounts, or all Macs, but the Reset Password Assistant is where you need to eventually arrive to avoid having to erase the Mac.
Read what is very likely to become necessary under "If no other solution is successful, you have the option to reset your password by erasing your Mac," but before proceeding with that ultimate solution then by all means contact Apple Support to confirm you exhausted all available options: Contact Apple Support. For example, they may suggest using another device to log in to your iCloud Account with your Apple ID credentials to confirm they are correct, and possibly use that option to reset your Mac's password. Presumably you used your Apple ID and its password to log into this site, so those credentials ought to be ok. Using them to reset your Mac's password may or may not be an option, but if it is they can talk you through it.
Good luck!
