So I had this problem about a year ago and it is extremely frustrating. What fixed the problem for me was removing any unions created during 2019 or earlier. Roblox randomly changes how they handle unions over the years and it causes conflicts like this.
How many tris in your Union, and how big is it?
Unions are messy versions of MeshParts, so you may want to convert it to a Mesh to decrease the amound of tris.
Send an image of the Union(s) you are building.
Just wanted to know how many tris are in your Unions. I guess if you can create the Union it should be alright to upload.
I just wanted an image to get an idea of how many tris could be in it if you didn’t have that information.
Unions were limited to 5000 tris according to Solid Modeling | Roblox Creator Documentation, but I think I read somewhere that it’s been increased to 10000 (maybe that’s MeshParts, I could be wrong).
Roblox studio has a set data limit that it allows uploading at any given time for every data file, unions in particular can be data heavy. When it say that your union has “Exceeded Limit” it means a union on the game has exceeded the threshold to send over to the server at the given moment. I have found that if you upload a game incrementally instead of all at once its less likely to error and you can debug what you did easier as well when the error does occur.
Incrementally just means as your making the map or item.