As you mentioned, this is indeed a version code thing, and is intended. The idea behind it is if a user tries to install an older version of the same app, the OS won’t let them. This is detected based on the app’s package name, which should be something like “com.mycompany.myapp”, and the version code.
To get around this, there are a few options:
- Uninstall previous app (as you mentioned)
- Sign the app with a debug key, which will signal to the OS that it’s alright to install “apk”s of various version codes. Make sure not to upload an apk signed this way.
- It’s been a while, but I believe if you use “adb” to install an apk, it automatically removes the previous version. I could be wrong, but worth keeping in mind.
Apart from these, there’s not much else you can do. This is intended, although I do agree the message “App is not installed” is not detailed enough.