I don't think I've heard of this before. It sounds like macOS is blocking you.
Try this, go to the Apple Menu (upper-left corner of your Mac) > System Preferences > Security & Privacy > General: make sure the radio button named "Allow apps downloaded from App Store and identified developers" is selected.
Another reason why you might see the "do not enter" icon over an application is if you're running a version of macOS that's not supported by Snagit. The latest version supports macOS 10.11 through 10.13, so if you're on 10.10 or older, you'd have to either download an older version of Snagit, or upgrade your mac.