Mailpoet responded me this, which I tried and solved my issue:
That file that’s missing loads and requires multiple other files which is most likely why this isn’t working for you. I believe on an update the files aren’t copying correctly on your install.
Can you try deactivating and deleting MailPoet (if you don’t have access to the site because of the fatal error you may need to remove the “mailpoet” folder via FTP). Then once removed try re-installing and see if it then works correctly?
