You are getting that error message in your KCB mobile baking app because you replaced your Safaricom simcard and the simcard’s serial number changed. For security purposes, if you install KCB mobile app, it registers your simcard’s serial number. If the serial number changes due to sim swap or replacement, the app won’t work.
You will need to visit KCB Bank Branch near you to have your app refreshed so that it cannot register the new simcard serial number.
I you have not changed your sim card, then make sure that you have updated your app to the latest version. Go to Google Play or App Store and download the latest version of the KCB mobile app.