You get this error when trying to update the signature for Mac Mail in Mac OS:
Please try creating a signature in Apple Mail manually in the following way:
- Start Mac Mail.
- Press Cmd + ',' to open the settings.
- Click "Signatures".
- Create a signature by clicking the '+' button and enter some test.
- Close the signatures dialog.
- Quit Mac Mail by pressing Cmd + 'Q'.
Run the "Update all signatures" from the Xink menu.
If you are still getting the error please check your iCloud settings as described here: