Submit UI Action is not available on the test steps in ATF to create test steps.
Release or Environment
Submit UI action on the table "sys_atf_step" might have been deleted. You can verify it from the sys_metadata_delete table.
If the UI action is deleted, you can find the record from the sys_metadata_delete table and restore it back. Else, you can import the Submit UI action from any of the OOB instances.
The Page that opens after you click on NEXT button on the ‘Add Test Step’ UI Action Page, is a record from table "sys_atf_step"