Android - Install button greyed out on play.google.com

Remove the "gl=US" at the end of the URL address.

The install button is greyed out if you have no compatible device linked to your Google account.

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. It seems that the web search found the US version of the play store, whereas when I went directly to the play store, it took me to the version for my local region.