I also turned on php error reporting, but it’s not posting any errors.
EDIT:
I removed the access restrictions, but still getting a 500. You can view the page & try to process a sandbox payment:
https://donations.gjt-sa.com/phpsample/
BTW, I added the SANDBOX_LOCATION_ID variable to the .env file since it was missing & I got Google Pay & Masterpass options showing up now, which they didn’t before.