Hello Roger Sinden,
If it still won't allow you to sign in on the iTunes Store app, you can try removing the account then signing back in. Depending on how old the software is on it, the steps may be different. On more recent software versions you would go to Settings > your name at the top > Media & Purchases, and select Sign Out. On older software versions, you would go to Settings > iTunes & App Store, tap on the Apple ID and select Sign Out.
Another thing that could be a factor is your Apple ID security type. Do you have the two-factor authentication security enabled on the account (where you enter a 6 digit code when signing into your account)? If so, depending on how old the software is, you may have to enter the verification code a different way. On newer software versions after you enter the password, you get a separate, specific window to enter the verification code. On older software versions you won't get that. If your Apple ID does use two-factor authentication and you're not getting a specific window to enter that code upon signing in, then that would mean the software on the device predates this Apple ID security method. If that's the case, you have to enter the verification code with your password. So for example, if your password is "Apple" and the verification code is "123456", you would enter "Apple123456" (no space between the password and verification code) in the password field.
Cheers!