Make sure you have extracted all the files from the zip archive into a folder. If you have Windows run the program as an Administrator. Alternatively, try another Extension Manager, such as ZXPInstaller.
Make sure you have a compatible version of Photoshop among those listed (CC2015, CC2015.5, CC2017, CC2018, CC2019). To find out, just go to Help -> System Information and read the first line Adobe Photoshop version: ...; if you notice that the installed version is lower than the CC 2015 version you need to upgrade to a newer version.