So, this needs some terminal knowledge to understand, so I'll do a quick explanation.
You must go to your terminal, it's an app on MacOS so just search it on the search bar ( duh :-: )
Then, you gotta write:
chmod +x <app-location>/Contents/MacOS/reky
To get the app location into the terminal, you go to the app on finder, then drag the application onto the terminal window, it should be something like /Applications/reky.app for example
So the total command on this example would be:
chmod +x /Applications/reky.app/Contents/MacOS/reky
This is because every Mac application is a folder with contents, and the true executable version of reky is actually hidden inside those folders.
Do take note, this is an example, so remember to put your real path. Also, if the path has a / already, don't add it again. This should fix the issue. You can close the terminal and use it normally. Enjoy :)
edit: Meant for edits341, I'm pretty sure the app creator knows this... or maybe not, just saying.