The app currently specifies Android 7.0 as its minimum, so we’d like to understand what fails before recommending an older version. Does Google Play refuse to install it, does it crash, or does it show a blank screen? Please share your phone model and OpenSprinkler app version.
Also, please update Chrome and Android System WebView to the latest versions available for your phone. As a diagnostic, try opening your OSPi’s local IP address in Chrome and let us know whether the browser interface works.