i can't connect to iTunes it won't let me sign in

To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

See the article https://support.apple.com/en-in/HT201400

Are you able to use the same ID to sign in here or to icloud?