It is possible that you searched for the app using google. Somehow the google result links to the US version of the page, which grays out the install link for non-US people.
Remove the "gl=US" at the end of the URL address.
In my case the button was greyed out when I accessed the google play store by using a web search for an app and then following the link to the play store.
If I went to the play store first and then searched for the app, there was no problem.
The solution proposed by Georgi121270 (remove the "gl=US" at the end of the URL address) solved the problem. ...
Here is how to fix the problem without root and without setting the volume to some predefined volume (such as 100%).
This "feature" is controlled by a System.Global property audio_safe_volume_state, which is also reset automatically every 20 hours (this is why the volume periodically gets lowered all of a sudden even without plugging / unplugging ...