Purchased a 30-day subscription for a 10-Player Realm via the Nintendo Switch. 

Attempted to upload a 288.6 MB world from a previous Realm. 

After remaining on the "Initializing Upload" screen for approximately 20 minutes, it gave an error that simply said "Failed". 

Since that time I, nor the people I invited to my Realm, have been able to join. We receive the error "Failed attempting to join realm. Please try again later."

Troubleshooting steps attempted:

Play > Edit Realm Icon > Reset World

Result: "Resetting Realm..." window opens, 60 seconds later: "Failed to reset the realm. Please try again later."

Play > Edit Realm Icon > Backups > Selected the only backup available and attempted to replace. 

After about 30 seconds: Replacing the world with the selected backup failed. 

Play > Edit > Download World 

A 0.20 MB file immediate downloads. The world opens as expected locally. 

Play > Edit > Replace World > Selected World from previous step

Immediate get an error that simply says: "Failed."

Performed all of the above steps on the Nintendo Switch. Then repeated all of the above troubleshooting on the Windows 10 version of Minecraft.

Attempted to connect to the Realm on the Windows 10 version.  It now lets me to connect properly. 

Save & Quit > Play > Edit > Replace World > Selected world I originally wished to upload

Upload was successful. Able to connect to the world on this Realm. Issue Resolved. 

Reporting as a bug to help others having this issue to see the steps I took.