If fields don't exist in the database that you specified, you will see an error screen
The error screen you will see if the field selection contains non-existing fields.
Study the errors in detail and see what went wrong. In this case for demonstration purposes I changed a few field names and of course that caused this error. My best option is to click the "Uncheck the Fields in Error" button and return to the field selection screen.
The fields in error are now unchecked.