Hi! Without knowing the error itself that appears to you, we can't know for sure what is causing your issue.
Out the top of our heads, the most probable causes are due to incompatible android versions or due to the apk signature.
If it is due to the apk signature, check to see if you have the option to "allow unknown sources" enabled on your phone (https://www.google.com/search?q=android+%252B+allow+install+from+unknown+sources).
However, if it is due to android versions incompatibility, then there is little we can do...
We've build the apk with Android 6 (API 23) as the minimum version (it is also the oldest one supported by Unity that we know off), so if your device is running an older version than this one, we can't easily help you.
There can be other causes for the error though, so if you could provide us with your device model and android version number, we could try and figure out what it could be happening.