Thank you for your post.
You have "5" currently stored in the clipboard, so when you tap paste, "5" now appears in the middle of the Container field.
Does this inability to add a signature only happen on this layout? That is, if you create a new layout with the Container field, does the issue still appear? If not, try replacing the Container field on the layout.
Also, in case memory in the application has been compromised, try relaunching FileMaker Go and see if the problem persists.
Let me know if you continue to run into difficulty.
To relaunch FileMaker Go, double click the home button and flick the app up from the resulting list of apps.
Might you be using a custom menu set here or a password with limited options specified in the privilege set's available menu commands drop down?
Thanks..rebooting did the trick.