Green light on your Z21 means it is in CV programming mode.
Are you trying to read the address of your loco using the Z21 App? This operation should be easy with the app. Just put your DCC loco on the track, go to "programming" then "Program Loco Address" and you should be able to read it.
The loco should make small movements during this process.
Once you know the address, you can create a new loco entry in your library with the same address and other details+photo
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