Failure [INSTALL_FAILED_DEPRECATED_SDK_VERSION: App package must target at least SDK version 23, but found 21

I have a feeling this is one of the first of many of these type of posts we're going to see. 

Android 14 is now explicitly blocking lower target build APKs from being installed on devices. The only "workaround" to this is a forced install through ADB. It can't be forced any other way. You will have to work with the app developer to have it updated to be compliant in order for it to be installed.

This will likely be a big problem for some enterprises that have a legacy enterprise application that was developed internally but no longer maintained that are forced into a device refresh to devices running A14+.