"Yo tengo que levantarme a las seis."
Translation:I have to get up at six.
Check levantar and levantarse in the dictionary. Your sentence doesn't say what you have to raise. Duo's reflexive construction indicates that it is yourself that you have to "raise."
Spanishdict.com will also give you the conjugation of levantarse, in case reflexive verbs are new for you. (Verbs used when the subject of the sentence does something for himself: brush your teeth, comb your hair, wash your hands, etc.)
Another help: studyspanish.com (Grammar Unit Five). Explanations, examples, free quizzes!
In this sentence I think of levantar as "to get up" not as "to raise" so "Yo tengo" would mean that "I have" to get up and the me seems redundant since I know it's me who is getting up.
Tengo tells you "I" am the actor BUT it does not tell you who or what is the object of that action.
Levantar == to raise or lift up or pick up something
It is a transitive verb; so, it needs an object to receive the action.
In this case, the object is "myself" --- in Spanish "me".