This error is often due to an older version of Document Template Manager being installed. You'll need to uninstall and then reinstall Document Template Manager on your workstation to make sure that the settings of your installed version match with the settings for the current version of GIFTS Online.
Under Programs and Features on the Control Panel, select Document Template Manager and select Uninstall. Delete the shortcut on your desktop, and then in GIFTS on the Admin ribbon select Install Template Manager and reinstall it.
If you click Install Template Manager and get a window which says "The Document Template Manager is installed on your system. Please run the application outside of the browser":
1. Right click on this text. A menu opens.
2. In the menu, click Remove this application... and click Yes to confirm you want to uninstall.
3. Then right-click in the windows again, and select Install Document Template Manager.