This has been happening since I bought my realm in march. It is extremely frustrating to not be able to upload a world to a realm I'm paying for monthly. I have asked for help and they just tell me to "try again" or check the FAQs. I'm getting very tired of not getting any real help, since I have been looking for months. Hopefully this gets resolved soon, and if anyone knows any tricks to getting it to work please lmk!!
ive been having this problem a lot on my switch, and before i had the same problem on pocket edition. But i tried just downloading from past saves of the realm on pe since i hadnt tried pe in a long time, and that worked. I only tried it once and i will no longer try and upload new worlds since im scared of losing mine.