If you attempt to delete a member type that has records attached, you will receive a message telling you to re-assign those records. However, if an attached records are inactive (deleted) you will not see them on the main members list.
To see these inactive records, follow these steps:
Click on the member type at the top of the members list to filter the list down by only records in that member type.
Click on the "Filter" dropdown tab
Choose "Inactive" under "Active in Quickbooks Online and the deleted records will show up.
Once you see the records, change their member type to non-dues paying member. To do this, go to the record and follow these steps:
Click on the Details tab.
Click on the member type dropdown.
Change member type to "non-dues paying member."
Once your deleted records are no longer a part of the member type, you will now be able to delete the desired member type.