How to obtain “passcode”?

Updating the operating system on an iPad or iPhone will prompt for the screen lock passcode. That is normal and you simply enter the (typically) 6 or 4 digit screen lock passcode (or password if you use a complex screen lock password instead of a numeric code).

There is nowhere for you to see your screen lock passcode unless you wrote it down somewhere. You either remember it or not.

Have you tried your iPad passcode? Or is it asking for a password? If a password, have you tried your Apple ID password?