I just spent hours troubleshooting this in a fresh install of SSRS 2019 using an updated database from a prior version.
I'm using an execution account with low permissions and did not want to make it a full Local Admin permanently but I tried adding it, unsuccessfully.
I removed the account from unattended execution in Report Services Configuration Manager and suddenly was able to export to XLSX. I was not sure what the fallout would be for existing reports, so I was not satisfied with this solution either.
I saw the recommendation here to use Process Monitor (procmon) to watch for registry key access denials.
It was difficult to search all of the output, I had tens of thousands of rows for a 15-second run while I duplicated the issue but you can filter by registry key access, then look for the RS executables or the result ACCESS DENIED.
In my case, it was HKU\\%service account sid%\Software\Microsoft\Avalon.Graphics, and granting the execution account (not the account that runs the service) Read access to this key resolved the issue.