"I had to be here."
Translation:Musiałem tu być.
Is "tu" always interchangeable with "tutaj? The app didn't accept it in my answer to this question.
In some senteces "tu" sounds better than "tutaj" and contrariwise. But the difference is difficult to explain. Generally, they are synonyms and using one or another doesn't make any difference in meaning so your answer should be accepted.
I think we sometimes use the rules for Jemu/mu cię/ciebie with tu/tutaj dziś/dzisiaj, but those rules are mandatory with pronouns and only a possibility with dziś/dzisiaj tu/tutaj