Hello Swipe! I Need help with Error Message: Default value does not match the field type - /v2/custom_fields/custom_headers

From the above message, I can understand that the field that you have created has a different field type than the input that you are entering in that custom field.
So, we request you to please edit the custom field and change the field type and try again.

To change the field type please refer to below mentioned article: