With the Customer-Details report, you can identify customers with zero class check-ins.
Navigate to Reports > Marketing > View.
Select the time frame (from opening day to today) > Export the report.
Under the "Total Check-Ins - All Time" column, sort/filter for zero.
Retention and Activity Segments
The Customer-Details report has the option to filter on Retention and Activity segments.
To find ‘New’ customers without a check-in, set the date filter to be the last 3 months and select 'More Filters' and set the Retention Segment filter to ‘New.’
If you are looking for customers with no check-ins and who created an account over 3 months ago, set the date filter to be past the last 3 months and set the Retention Segment filter to ‘Dormant.’