AP StyleGuard is an automated style-checking tool for Microsoft Word or Outlook, developed by Tansa in collaboration with The Associated Press.
If you had AP StyleGuard installed and upgraded to Mac OS High Sierra, the version of the program you are running is no longer compatible due to Apple development changes.
You simply need to install a newer version of StyleGuard to get it working again.
We have updated our OS X 10.10 Yosemite and later (Word 2011 or 2016) installer, available on our website here: https://styleguard.com/downloads/
Please click on that title, and instructions to uninstall and reinstall will appear. Our updated Yosemite installer is now compatible with OS High Sierra.
You may need to bypass Apple security during the first launch after reinstalling. Simply press the control key while clicking on StyleGuard in the applications folder, then select open from the menu.