Why can’t I use my Apple ID in iTunes on my iPhone?

Difficult to say why as you haven't given any information. What are you trying to do with the account e.g. log into it, buy something ? What happens when you do it ? If you are getting an error message what does it say ? What have you tried to see if it fixes whatever is/isn't happening ?

Try a restart.

Try a forced restart.

Reset Device

Reset – iPhone 7

Hard reset