[Appointments+] Cannot get Appointments+ to work with Google Calendar

I've been trying for 5 hours to make Appointments+ connect to a Google Calendar.
Followed all these steps: https://premium.wpmudev.org/forums/topic/appointments-google-calendar-api-1
I've reset the service account key etc multiple times. There are no blanks added to the key or secret.
Both main domain and subdomain ownership have been verified with Google.

Calendar is public and has been used fine with other applications.

I am completely stuck. Support access has been granted x 5 days.

The website in questions is .
wp.interiorhorsecouncil.com

Thank you for your assistance. I'd really appreciate a hand here.
Diana