Unable to access iCloud on my old MacBook Pro due to unknown password request

After probably 3-5 years, i've logged in to my old Macbook Pro (the only Apple device i have left appart from my wife's macbook air, where i also logged in once), which my mother is now using. In my account, i was logged off from the iCloud. It first asked for a password of my account (fine), the Apple ID password (also fine), and then it asked for my 'Macbook Pro - Pro' password, which I have no idea what this is. This is the only Macbook I had, and I'm already logged in to it. Is there a way I can restore this? All I wanted to do, is restore an iPhone backup (also and old one, like over 7-8 years old), which I cannot do without it.

[Re-Titled by Moderator]

Original Title: iCloud sign in issue