Still not working, using Nintendo on 1.16.1 today cannot upload a Realms Plus world to a realm. I have even created a new realms plus subscription and I get the same "An error occurred" after trying to create from a template.
If I create a world locally it works fine, but replacing a realm with that local working copy also generates the same error message "An Error Occurred" . Access such realms from another account just has user drop into a vanilla world.
SORT THIS OUT MOJANG!!
I can confirm that if you can log into your Ninentendo account on Winodws 10 - using the xbox app and create/upload realms from Win10 to realms, the operations all complete successfully. But errors still persist when i try it on Nintendo.
Not everybody has Win10 - so this is s rubbish work around
Cmon Mojang FIX THIS!