This error happens when the form name is in use. 
  1. Navigate to Site Explorer > Forms
  2. Click the Search tab and search for the form name you want to use
    • If the form is returned:
      1. Click the hand icon to edit the form properties
      2. Update the form name
      3. Click Save
    • If the from is not returned:
      1. Select the Deleted Forms folder
      2. Search for the form name you want to use
      3. If it is found, permanently delete the form in order to use the name