Description
The Calculate Field tool cannot be used if the input feature service is in a current edit session, or if you have enabled the Enable Undo toggle button on the geoprocessing tool dialog box.
This limitation is only true for a nonversioned edit session; Calculate Field can be used on an input feature service in a versioned edit session.
Solution
Save or discard your edits before running the Calculate Field tool or turn off the Enable Undo toggle button on the geoprocessing tool dialog box.