How to get into Serbia?

What do you mean? Serbia is open to all travellers, I only think you'll need a PCR test. 

There are no trains from Budapest to Belgrade. Zagreb to Belgrade currently has one direct train per day running. Trains between Montenegro and Serbia are back again, with the night train as well as the daytime train.

Hello, thanks for your answer.

I would like to go to Beldrade, coming from Budapest but indeed the interrail website can’t find any trains

i am going to check a train that would go to zagreb but it is more complicated 

Thanks for your help

Bérénice  

Hey!

You can make a mix of train+bus from Belgrade to Budapest.
Train goes until Serbian border city of Subotica (which is also worth half a day stay over) and then take a bus from there to Belgrade.
@jonworth on twitter did this trip recently. You may check it out there.