Android developer verification - Register APPs not on Google Play

I am trying to register an APP that is installed outside of Google Play Store and and it wants me to select a eligible public key.

But the key that is showing isn't the one that this app uses.

And it will not let me select another one (or rather when I select Change Key there isn't another to select when I use the search)

Where is Google getting the certificate fingerprints for APPs that are not on Google Play? The App I am trying to register is strictly on my devices.

Hmm,
Google has confirmed that ADB (Android Debug Bridge) installations are exempt from the mandatory verification check.

Also,

• When the warning appears, you shouldn't just look for an "Install Anyway" button (which Google often hides).
• You may need to go to Settings > Security > More Settings > Developer Verification (the path varies by manufacturer) and enable a "Trust Unverified Apps" or "Maintenance Mode" toggle.
• Note: This often requires the device to have Developer Options enabled first.

Buy hey, the above might not work for you either.

Anyway, I'm off out for a night out now, so good luck

Not sure what you are talking about this is how the screen appears to me