Description
Schema changes, such as adding or removing a field from a table, cannot occur while the data is being edited. The tool or the option used doesn't support operation in an edit session.
Solution
Complete your edits; then run the tool. To save or discard edits, on the Edit tab, in the Manage Edits group, click Save 
or Discard 
to save or discard your edits, respectively. See the tool's help for more information.