Everytime I try to change my Realms world to another different world that is modded it does not work, It keeps saying error occured

Everytime I try to replace a world in my realm with a modded one it doesnt work I've tried every way I could think of to get it to work but it hasnt, the only way it lets me change the world is if it doesnt have mods, which is kinda frustrating. Everytime I do try to change it to modded world it says Error Occured after saying Export or import succesful, then it loads all MB only to go  1 or 2 MB over how much it actually is. Please fix this to where I can play with modded realms.