Multiple damage types for NPC Attacks

I was trying to create an NPC and the attack I was trying to set had multiple damage types and dice. I could only select one of each, and if I tried to add both dice types to the attack damage calculation, it cut off the 2nd dice, i.e. 2d6+4 bludgeoning damage + 2d4 radiant damage would result in 2d6+4 bludgeoning damage

Which game system / specific sheet are you using? If you're talking about the 5e 2024 sheet, it is possible you just have to do it from the standard or compact views on the NPC sheet, not the statblock view. Settings button top right of the sheet, change the layout to Compact. Then edit the attack and you can change back to statblock after. If you're using another character sheet there's probably a way to add the other damage, but you'll have to specify which sheet you're working with.