Description
Schema changes such as adding or removing a field from a table, can not occur while the data is being edited. The tool or the option used doesn't support execution 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, Save 
and Discard 
save and discard your edits, respectively. See the tool's help for more information.