ATF: Browser failed to submit form to server when clicking UI Action

When running an ATF Test, it fails to complete the Test Step, when submitting a form to the server after populating some fields on the form. For example, when trying to Save the Incident form, the Test Step fails with the following error -

Browser failed to submit 'incident' form to server when clicking UI Action 'Save'. 

The form contains mandatory fields that have not been populated in any previous ATF Step. For example, on the Incident form, when the Category and Sub-category fields are mandatory and only the Category field is populated in a previous step, it will throw this error because the form cannot be submitted if a mandatory field has not been set a value for the ATF Test.

Ensure that all mandatory fields on a form are populated in an ATF Step before submitting it via a UI Action such as the Save button.

Test Step Categories - Form category [Madrid]