Check your graphics card drivers are up to date, check there are no anti-virus programs stopping it, try lowering graphics settings, run as administrator. If none of those work then i'm not too sure what the problem/solution is.
One thing you could possibly try, see if you can load up a different game that's similar to this and see if it works fine, like VRChat or something like Sizebox which i think use the same engine/assets, see if it's just this game that is having issues or if other similar games are also failing to load properly.
I've tried opening Flattened HD, fail.
I've tried opening Sizebox. Success, takes a couple of seconds but that's the content I have.
White screen is really annoying and is a blue screen essentially
Edit: Should mention I've tried both multiple times, and that I've tried a lot of troubleshooting. Nothing works to fix Flattened for me.
Edit2: And I only remember now that the previous version of Flattened was a flawless open
Update on this, I had a download of Flattened.rar in an itch file, unpacked it and it was missing a file, something like resources.resS
I have no idea if it has something to do with the game on a white screen.
Something else too is that I uninstalled and then installed Flattened again, tried running it up again, and the screen was grey, but then white and... forever unusable.
Deja vu... nearly forgot to mention its only a data pack, no game