Orchid POS will only allow you to delete a customer record if there is no sales history associated with it.
To delete a customer, go to Customers > Search all > Search.
Search for the record, then use the delete icon under the Action column on the far right.
Orchid POS will confirm you'd like to delete the record. If there is sales history associated with it, it will then prompt you to merge the record with another. See Merging Duplicate Contacts for more information.