Email being used (error message)
Each E-Signature user on your site must use a unique email address in their E-Signature settings.
If you're seeing this error message, it's because another WP user is utilizing your desired email address in their E-Signature settings already.
These are different than WordPress account settings and refer to the email address that is entered on a user's WP E-Signature settings page, shown here:
To see which E-Signature user is already using that email, you'll need to check with the other E-Signature users on your site.
Each user can visit E-Signature > Settings > and see if the desired email is in the settings. Once you find the user with your desired email, they should save a new email address in their settings to "free it up".
Then you can save your settings with the desired email, which is now unique to your user.