Is it possible that there is no verb in a sentence?
Take the following sentence as example:
Morgen muss ich zum Arzt.
Does this sentence make sense?
Like in English, there are similar modal verbs (Modalverben) in German. These are dürfen, können, mögen, müssen, sollen und wollen.
Mostly, they are "modelling" the main verb but it is also possible to use them without main verb.
In your example,
Morgen muss ich zum Arzt
the second word, muss, which is the conjugated version of the müssen, is the verb of the sentence. Like Torsten wrote in his comment, you could also say
Morgen muss ich zum Arzt gehen
and in this case, the meaning would be almost same.