Access Form: New Record command not available now
The fact that the error message said "acNewRec command is not available now" should have directed you to the line DoCmd.GoToRecord , , acNewRec.  The form's recordset is not guaranteed to be open and positioned correctly during the Form_Open() event, especially not always accessible via DoCmd methods.  Documentation says otherwise, but experience shows that there are potential problems with relying on the recordset to be in a reliable state in Form_Open.  

There are other ways to make the form automatically open on a new record.  Try putting the same line of code within the Form_Load() event.  Setting the form's Data Entry value to Yes allows only new records.