so i tried to apply a rune to my enchanted diamond sword and it wont let me add it at all no price shows no end result nothing just a red X

i am also having this error on 1.16.1

Can you get me some images and a log of exactly what's happening?

this sounds to me like your item is so enchanted that the exp cost is too high to repair/combine/enchant the weapon further....try it in creative mode? see if the error still happens? vanilla minecraft anvil doesn't let you enchant an item if the cost is over 38 levels in survival mode. that's what gives the Red X in a vanilla anvil.