After several failed attempts I’ve managed to get a virtual sensor/switch to appear under Device in “When this happens” in Alexa routines. I had to log out of everything after completing the instructions and disable/enable Smartthings skill in order to see it appear.
I followed this https://thesmartcave.com/smartthings-with-any-alexa-device/
I found when I selected the device under “Choose Device” - it says
“Error This device is not currently supported” . The solution is, to quote @JDRoberts (to whom much thanks is due for other posts)
“Assuming that you are in the US or the UK, If you read the thread above from the top you will see that a number of other people had the same problem. All you have to do is disable the smartthings skill, sign out of the Alexa app, sign back into the Alexa app, reenable the SmartThings skill, and everything should work.”
And it’s now working: 