I wanted to know to complete the Amal for 15 Rajab do you have to keep a fast? Or can the Amal still be done without fasting?

All the Amaal and Fasting in these 2 months Rajab and Shaban are categorized as Mustahab act ( I mean recommended) so for if any any one wishes to fast any day out of these two months can do so and inshallah he/she will be rewarded. As far as performing an Amal is concerned, we have different amaal for different occasions but still no matter if somebody has missed out can perform any time. While it’s clear that we can’t compare the sawab of amaal done on that very day or night with if we perform it after passing the event. But as these amaal are Mustahab one can perform later on too and inshallah will get sawab. Allah(swt) knows better.