The new MacOS 10.14 Mojave has come up with an update to restrict it's access for third-party applications, especially when it comes to Mails, Contacts and the Calendar. The users will thus need to allow Mail Extractor Pro for permissions to access your Mails, Calendars and the Contacts.
Here is the step-by-step procedure for the same.
1- Please go to the "System Preferences" of your Mac,
2- Open "Security & Privacy" from the top
3- Click on the 4th Tab "Privacy" which is after Firewall
4- Then Click on "Full Disk Access".
5- Add Mail Extractor Pro from your Application folder
6- It will prompt with a pop that you need to quit Mail Extractor Pro. Now, click on "Quit Now."
7- Launch Mail Extractor Pro Again, and you will be able to view all the folder from Apple Mail for conversion.