Can't run an apk file

Yes, I know this is an old question but I haven't found an answer yet and I think finding it might be useful to others, too.
I have an HTC One M7, running 4.4.2 KitKat.
I'm developing an app and want to test it but I can't even run it on my own phone!
It installs ok - but when I click on the...

Can't run an apk file

Can't run an apk file