I figured it out. The line that says you need camera permission was being used as a filter in the google play store:
<uses-permission android:name="android.permission.CAMERA" />
I don't know why this was the case because all of the phones / tablets that didn't work already had a camera... but the fix was to change it to the "uses-feature" tag instead and not make it required:
<uses-permission android:name="android.permission.CAMERA" />
Hope this helps someone