Two features to check:
Did you set up the layout with a custom menu?
Have you set up and are using an account/password with a less than full access password.
Either can result in being unable to insert into a container field.
Thanks for the answer
Seemed that the custom menu was causing the fault added the script step to go back to the standard menu and then after the signature is created have a script trigger to revert back to the custom menu. This baffled me for weeks. Much thanks for the answer