I'm also using OnePlus One running Android 8.1.
Same issue since many days. Now can't sync my passwords running Enpass 6 on Desktop. Android still running Enpass 5.x version and not syncing due to database incompatibility.
This is what I see on my Play Store when I open https://play.google.com/store/apps/details?id=io.enpass.app in Playstore App
And this is what I see on my Desktop (and again before you mention that it says "Installed" - it's because I'm running older version 5.x of the app):
Now please just don't say that everything is good and we should be able to install it without trying even to find a root cause.