This is a known error that we're working on. Unfortunately, I don't have a timeline for a fix yet. I'm sorry for any inconvenience.
I would recommend continuing to try the set request form when you have a new request, but if it doesn't work the first time, then please feel free to post on the chat board thread about what sets you would like to see next, as I do add those suggestions to the list.
I see that the image you posted is for the Astral Radiance set, that set is coming soon!
Here is a link to the chat board thread I mentioned: