Cannot uninstall app via adb

I'm trying to update an app on a smartwatch, but whatever I try, I ran into errors. 1) adb install newversion.apk => [INSTALL_FAILED_VERSION_DOWNGRADE] 2) adb install -r -d newversion.apk => [INSTALL_FAILED_UPDATE_INCOMPATIBLE] 3) adb uninstall {packagename} => Failure [DELETE_FAILED_INTERNAL_ERROR] 4a) adb shell pm uninstall --user 0 {packagename} => success 4b) adb install newversion.apk => [INSTALL_FAILED_VERSION_DOWNGRADE] App is a launcher. OS has no launcher now. I still cannot install my new version. Checking for other users: pm list users -> users: UserInfo{0:Owner:13} running. So I'm the only user. Also tried the uninstall --user command with other user IDs like 0, 1, 10-15, 2000+, 300+, all nothing. I am not rooted. su does not work. It's an MT6739 chipset with Android 7.1.1 running. Nothing that I try lets me install my new app. Also not after factory reset.