I'm also experiencing this issue. I cannot upload a new world to my Realms server from either my Switch or my iPad. I get the same error everyone else here is reporting.
I have been following this issue for weeks now and there has been no resolution and the suggested workarounds are not working. Can we please have an update from the developers as to the status of this issue and a possible ETA as to the solution?
@Mega_Spud
Just upgraded to 1.16.2. I am able to upload worlds now. Even through the system still shows an error dialog, the world does seem to get replaced. The new problem is that upon quitting the game, the entire Switch system freezes for anywhere from 5-15 minutes. It requires a hardboot to bypass this wait. The Switch needs an emergency patch to fix the freezing problem.