There was someone a few weeks back who said the same thing. Turns out they kept tapping "install" multiple times and it just didn't work. Depending on the app, it may take longer to install than on the stock OS because GrapheneOS disables just-in-time compilation and replaces it with ahead-of-time compilation, which means everything is done on app install. It means the app install is slower, but improves security.
Enlightenment Not saying that you're doing the same, but a suggestion would be to tap install once and then wait for the install to finish to see if that helps.