The email error message may not be accurate, it appears the PDF merge is failing, and not an activation issue.
Try unchecking the “Send Email Attachment” option on all unused, unfinished or test Field Map Layouts or better yet, delete them. Then test the merge from the plugin’s admin screen, if it works then test the email attachment.
Or, if you’d prefer us to take a look, please complete this form to provide access to your installation.
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