I am having the exact same problem. It has been 4 days now of trying to replace my world on my realm, the world size is 582.5MB and even though it sounds like a lot this is not the problem as I have managed to replace the world prior to this "error". It would help actually if we were provided with more information on the error other than "Failed An error occurred". This could even just be an error code or a brief hint to what caused the error e.g. Upload speed. I feel as if I have wasted my money and am understanding of the increased work due to lockdown. Please get this fixed or even provide feedback ASAP.
why have you made realms purchasable if they don't even work. That's me had this same problem twice now and both times is soon after i buy a realm.