Welcome to the Make community!
So you want to build a appointment booking software like Cal.com, Calendly, TidyCal, Google Calendar, etc.?
I strongly recommend just using one of those services instead because allowing users to grant you access to their calendars is very technical.
Or you could just get the Free/Busy info from your own calendar to show them a list of time slots they can book.
For more information, see Check calendar for blocked dates on the landing page
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Investing some effort into the Make Academy will save you lots of time and frustration using Make.