Yes, but the meaning changes slightly.
The reflexive version, "du må bestemme deg", implies that you have to make up your own mind about what to do. "Du må bestemme" without the reflexive pronoun would be be used in cases where you had to make a decision on behalf of others as well, not just yourself.
If you translate it word for word you get "decide yourself", but since "to decide" isn't considered a reflexive verb in English, you would be wiser to leave out the reflexive pronoun in the translation.
If you translate the reflexive pronoun literally, you add a stress that isn't present in the Norwegian version, as the pronoun is merely a part of the verb. In Norwegian, the word "selv" would be used to stress that the decision was in fact your own to make.
Because the Norwegian sentence does not specify the "by yourself" part.
"Å bestemme seg" is a reflexive verb, so the "seg" is just a reflexive pronoun which forms part of the verb, it doesn't imply that you have to reach the decision completely without outside influence, even if the final decision is yours to make.
"Du må beslutte" can be a translation of "You have to decide" but means something different than "bestemme deg". "Du må beslutte deg." is not correct. "Å bestemme seg" is a phrasal verb, or more specifically, a reflexive verb, with a specific meaning, while "å beslutte seg" is not.