Since you say the list already existed and the form *was* working correctly, the only thing that comes to mind is maybe a Content Type derived from the Task content type was used for the list and now the form believes that it needs an outcome control in order to work correctly. 

Is this now happening to *all* of your forms across your entire SharePoint installation? If so, I recommend contacting Nintex directly for assistance, as this seems like it could easily be a bug.

I am adding this for those who get the error and have a task form.

You can find the "Outcome" control in the control panel.

Hi,

I had a similar issue with error message on the New Response Form on SharePoint online.

"ERROR_TASKFORM_SUMBISSION_WITHOUT_OUTCOME_TEXT"

My list has a choice column with name, Outcome and this is shown on the Nintex form. It appears that the Nintex script is assuming this is related to a Task form and expecting a value to the task outcome.  After renaming the column with “ReviewOutcome”, the error is gone. 

I understood that some internal names of user’s SharePoint columns are same as the internal names that were being used by Nintex scrips.  

Cheers!!!