Realm replacement not working

Ok, I have a survival world and I also have a realm. I want to replace the survival world with the current realm. I go to the realm and press replace. it then says export and progress and the green bar goes all the way over. Next, It says initializing upload and it is blank for a few seconds, later, it says an error has occurred. This has only happened to me in 1.15 and 1.16. If someone has a fix please comment! Thanks