Google play says "This app may not be optimized for your device", when I access app page with my tablet
This is the same as the old "Designed for phones" message. The guidelines for getting your app marked as good tablet app quality are here.
You need to submit a least one 7-inch tablet and one 10-inch tablet screenshots in your store listing on your Google Play Console.
For your app to be showcased in the 'Designed for tablets' list in the Play Store, you need to upload at least one 7-inch and one 10-inch screenshot. If you previously uploaded screenshots, make sure to move them into the right area below.
It's not clearly written that this is related to the message `This app may not be optimized for your device. But I have submitted 7-inch and 10-inch screenshots and as soon as the update were live, the message disappeared.
Google's quality guidelines say "For apps targeting minSdkVersion value less than 13, a <supports-screens> element must be declared with both android:largeScreens="true" and android:xlargeScreens="true"." This goes into the <manifest> element of AndroidManifest.xml, e.g.:
<manifest xmlns:android=….. <supports-screens android:largeScreens="true" android:xlargeScreens="true" />etc