HTC M8 won't install apk files

[cheri279]

Hi I need to install an apk file on my HTC M8. I recently upgraded to Android 6 and it now won't let me install. I have changed the security settings to allow unknown sources. When I download the file, the install button doesn't work. Any ideas?

 

[cheri279]

I solved the problem. I had Twilight running and when I paused it, I was able to press the install button.