@Wayne-TP hi Wayne.  No error and the voice command was recognised (Alexa said "ok") but did nothing. The light will also not switch off via the Alexa app controls.
However, here is some new info:  If the manual switch attached to the S112 is in the 'down' position, Alexa can't switch off. Conversely, if the switch is in the 'up' position it works fine. So this might be user error in not understanding how the manual switch is supposed to work. In my defence though, the TAPO app can control the switch correctly irrespective of the manual switch position. 
I'm going to reconsider the switch in Alexa today and retry......