title: APK files not install on my oppo a37
body: <p>I made a simple app by using android studio &amp; build both signed and unsigned apk files. Then put those files into my oppo a37 device and try to install those. But both files gave following error message.
  "Unable to install because of an error in the app APK. We recommend installing only certified apps from the play store."
  I already checked these APKs by using android studio emulator and both works fine...
  Please help me... Thank you...</p>