Hi there !
If you are trying to restore from an iTunes backup and it requires a password, that means that, somewhen since you connected this iPhone to your computer, you ticked "encrypt backup" and entered a password twice to be saved.
Here is an article to follow :
https://support.apple.com/en-ie/HT205220
I recommend you to try all passwords you can think about as there is no locking after a certain amount of attempts. Could be :
- session password
- apple id password
Please notice that it can be any number of characters, numbers, as there is no restrictions when creating this password.
To finish, if you can't find it at all, try to restore from an iCloud backup instead, if you have one, or from a non encrypted backup.