Why Install Fails
Android refuses to install an app over an existing copy signed with a different key. Patched copies are signed with your keystore, so a patched copy built with one keystore cannot replace a patched copy built with another. The installer answers with the short message app not installed, and no other detail, which makes the cause easy to miss. This page is part of the Morphe app for Android patching collection.
The same error appears when the original app is installed with the same package name the patched copy wants, or when the downloaded file is incomplete. A half downloaded file fails with a parsing error instead, so a clean app not installed message points to signatures first.
The Fix
Remove the older patched copy for that target app, then install the new patched copy again. Do not remove the original stock app, because the patched copy uses a different package name and does not clash with it. The clash is only between two patched copies signed differently.
After that, keep one keystore for all patches. Export it, back it up somewhere safe, and reuse it for every patch run. One keystore means every future patched copy installs over the last one cleanly, and updates stop failing with the same error.
For the next step, see Morphe Patch Sources Explained in the guides, which continues this setup from where this page leaves off.
If It Still Fails
If the error survives removing the old copy, check storage and the file itself. A full phone cannot install anything, and a patched file that was moved or truncated cannot install either. Patch again, keep the finished file on the phone until it installs, and check that the installer permission for the app opening the file is allowed.
Restart the phone once if the installer seems stuck. A stuck installer service can reject a good file, and a restart clears it faster than any setting change.
Related reading: Morphe SponsorBlock and Return Dislike, which covers the neighbouring part of this workflow.
Conclusion
App not installed means a signature clash in almost every case. Remove the older patched copy, keep one keystore, and the error goes away for this install and for every update after it.