Why does the message “App not installed because the package conflicts with an existing package” appear?

This message usually means that Android has already detected another installation of the same app or a conflicting variant. Android can only have one app per application ID installed at a time, and a direct update works cleanly only if the application ID and the signature match. Typical causes therefore include:

• an older Play Store version,
• a free, trial, or legacy version,
• remnants of a previous installation,
• or an additional copy in the Samsung Secure Folder. 

The cleanest solution is:

1. uninstall all older variants,
2. on Samsung devices, also check the Secure Folder,
3. optionally clear the cache of the Package Installer,
4. restart the device,
5. and then install only the desired version again.

If you switch between the Play Store version and the direct-download version, uninstalling first is often the safest approach.