"Some of the selected content couldn't be applied" When trying to add an addon to a realm.

Whenever I try to add a mod to my Minecraft realm, I get a message saying that some of the selected content couldn't be applied. This glitch has been in the game for months and still hasn't be fixed. It is especially annoying for those like me who bought the realm to play a modded world. But now I'm stuck paying 10$ a month for a useless realm. I spent hours trying to find a way around it. Every now and then I get lucky and it works. But only hours after I initially put the addon. This game actually makes me so mad. They always are working to fix the most random, rare bugs but never focus on the largest bugs in the game.

If anyone knows a workaround, please tell me!