Threema cannot be installed Sailfish 4.0.148 Koli

Hello on my newly set up Sony XA2 with 4.0.148 Koli I can not install threema.apk . Clicking on it only shows open. More does not happen.

I have also read this link.

But I do not understand what is written there. I do not know how to access the hidden files.
Can someone give me tips.