What is the difference between a Mac Password and Passcode?

I am just trying to connect an old iPhone 4s to my iCloud account and I was asked to enter my Mac passcode. I entered my Mac's password but it kept getting rejected.

I have never set up a passcode on my Mac (Catalina) and when I searched for the difference between a Mac password and passcode, everything seemed to link to Screentime which I do not use - I am the only user of my Mac. When I tried to turn on Screentime just for the purpose of getting a passcode, I was told that my account had to be converted from an admin account to a regular account - which is clearly not a good idea as the sole user.

What am I supposed to do?