Hard to say why. I do not have any machines running High Sierra, but that command is a very generic (non-macOS specific) command that should work on any version of macOS (and Linux, but Linux doesn't use *.app files). It is just setting the actual program to be 'executable' since the zip file messes that up.
Some things to try:
- Drop the `a` and do a `chmod +x /Applications/Digital\ Logic\ Sim.app/Contents/MacOS/*`
- Validate your path by doing a `ls /Applications/Digital\ Logic\ Sim.app/Contents` and seeing if that returns a "No such file or directory" (which would indicate your app is not living in your applications folder. you can adjust the path of the command to match wherever yours may be)
One final note is that you should not be including the backticks ( this thing -> ` ) in the command. Not insinuating you are, but just being safe :)