I'm having a problem uploading a theme builder json file

Hi there,
I just a theme from the divi marketplace and I am having trouble, I am unable to upload/install the divi-theme-builder.json file. I will provide all details here - I hope someone can help. I imagine others might be having a similar issue. This is not a library issue, it is a theme file for the global headers/footers.
* This was a fresh wordpress install. The website is www.musicforasong.com
* I was able to install Divi 4 and the main theme successfully.
* I was able to install the divi-theme-option.json file.
* I was able to import the demo content successfully
* I have updated the PHP file to the recommended server requirements (above them actually)
* I thought it might be a server issue, so I tried uploading this file on other divi websites at TWO other hosting accounts, and the same problem persists.
* I also tried downloading the whole theme again, extracting it newly, just in case the file got messed up while unzipping. Still nothing.
* The most bizarre part of this is that I do not get an error message - only a red circle with an X in the middle flashes briefly (instead of the green check mark of course).
* My hosting account support says everything is fine on their end.

Any suggestions?
Thank you in advance for the help!