Running version 2.12.06
There is a bug in switch-wizard-editor-rules.js which prevents switch key assignments to be made. When creating any key assignments through any of the create* methods, the code looks for a sensor in app.sensors using the uniqueid field for lookup. There is no uniqueid field though, so let sensor will always be undefined and the code for creating the assignment fails. The correct lookup field is called id.
The following fixes it for the time being:
cd /usr/share/deCONZ/webapp/pwa/js
cp switch-wizard-editor-rules.js switch-wizard-editor-rules.js.bak
sed -i 's/uniqueid/id/g' switch-wizard-editor-rules.js
Cheers, Uli