Diagnosis
The smart home platform can't see the plug because the Kasa skill isn't properly linked or the plug isn't in the same Kasa account you used for linking.
How to Fix It
Re-link the Kasa skill. Go to your Alexa or Google Home app, disable ("unlink") the TP-Link Kasa skill/service, then re-enable it and log in with the exact same Kasa account email and password used to set up the plug. Discovery usually works immediately after.
If That Didn't Work
Ensure the plug is online and responsive in the Kasa app itself first. If it's offline there, fix the WiFi connection. Smart home platforms can only control devices that are online in the manufacturer's app.
Heads Up
Never use the "Add Device" scan in Alexa/Google for these. Always set up the plug fully in the Kasa app first, then link the accounts via the skill. The platforms don't set them up directly.