the id is composed of the URL scheme “x-apple-reminder://” and the UUID.
You have to write
set UUID to "68B7CCEA-BE79-4602-83EB-9F28812A44A4"
tell application "Reminders"
delete (1st reminder whose id is "x-apple-reminder://" & UUID)
end tell
or still easier
tell application "Reminders"
delete reminder id "x-apple-reminder://" & UUID
end tell