If you receive this error while editing a form, please take the following steps first if and only if you do not need to collect contact information on your form:
1. While in the form designer page, go to the dropdown menu at top left > select Organization Primary Contact > Unmark the checkbox for Organization Primary Contact is Required if marked > click Save
2. Return to the dropdown menu at top left > select Request Primary Contact > Unmark the checkbox for Request Primary Contact is Required if marked > click Save
3. If the Contact field in question falls in neither of the above categories and you continue to receive the error, you must check every field in the form - if you are using any Contact-type field in the form, it will need the Last Name as an anchor, only by removing those other Contact-type fields will you be able to remove the Last Name field.