If you are trying to install some android application which is not downloaded from Android Playstore but has been shared by someone as apk, there are higher chances you may get a below error, while trying to install using adb install,
$ adb install YourAndroidApp.apk
2712 KB/s (7266004 bytes in 2.616s)
Failure [INSTALL_FAILED_TEST_ONLY: installPackageLI]This means that, the shared application has some test packages, so unless those has been removed and source is recompiled, you will not be able to install this apk. But adb command provides a flag “-t” using which you can install the apps with test packages.
$ adb install -r -t YourAndroidApp.apk
2566 KB/s (7266004 bytes in 2.764s)
Success
Thank you! That helped me a lot!