I’ve noticed on android, if you try to manually install an app that is already installed, android will put the message “App is not installed”. This can happen if the app you are trying to install is an older or the same version as the already installed one.
Did you try uninstalling any the existing app, assuming there is one and that’s what you experience? If that’s what the issue was, then keep reading for the right long-term solution. If not, maybe it’s something else I’m not familiar with.
Android has a variable called “versionCode”, that tracks if that build is newer/current/older than other builds of the same app. If your app is already installed with a “versionCode” of 5 (for example), and you try to install a new build of the same app with “versionCode” 6, then android will understand the new build is an update, and will update the app.
If however the new build has “versionCode” of 5, or lower, android will not change the already installed app, and display the message “App not installed”. I find that a bit misleading, as it would be better if it displayed the reason it’s not installed, but I’m sure there is a reason behind it.
For more information you can read Android Developers - Version your app.