The missing verb is llover, which means “to rain.”
Your sentence translates in English to this.
When it rains I have no desire to go, but not going to rain tomorrow.
If I understand you correctly, you need to determine what form of the verb “llover” will go in the blank in your original sentence.
In this sentence, you are looking for the proper form of the verb “to rain” for the meaning “it’s not going to rain.” In this case you need an unconjugated verb. The unconjugated form of the verb in English is “to rain” and in Spanish is “llover.” Notice that in this verb the "o" changes to "ue."
Cuando llueve no tengo ganas de salir, pero no va a llover manana
It is in infinitive form because the word va a is already conjugated.