If you get an error while trying to save an Activity in the CRM (with the Appointment box ticked), there are three ways to troubleshoot this;
Untick the Appointment box in the CRM so the activity will save and you don't have to enter the information twice
Log out of the CRM
Log back into the CRM and select 'Allow' to the Calendar Access notification
Search the Activity, Edit, tick the Appointment box & Save
2. You aren't subscribed or don't have access to edit this person's calendar
Untick the Appointment box in the CRM so the activity will save and you don't have to enter the information twice
Go into your Google Calendar and first of all check that this person appears in your list of calendars (down the left)
If it doesn't, you need to subscribe to the calendar, using the + icon next to the heading 'Other Calendars'
If you don't know how to check whether you are able to make changes to a calendar, you can click on the three dots and go to the 'Settings and Sharing' to check that you're loaded as 'Make Changes to Events'
If you're either not able to make changes to their calendar or don't have access to their calendar, you need to discuss this further with the assignee and get them to share their calendar with you.
If there are any other accounts listed here (when you're in your Gmail or Google or Calendar) log out of everything.
The CRM can get confused with conflicting domains so you don't want to be logged into multiple emails on the same 'window' that you're using the CRM on.
If you need multiple email addresses open, we would suggest creating Google People - this keeps History, Bookmarks and Passwords separate (For Example. One personal Google Profile, one Work Google Profile to keep domains separate). For further information about Google People, refer to the Google Person section under 'Gmail Setup Tips'