I faced the problem while using Multi-configuration project and specifying API as one of the axis:
When I specify API as list of numbers "15 19 21 23" and it works fine - AVDs are created and fired up.
Next I tried to extend API levels to test and tried API axis as "15 19 21 23 24" - it fails, the message was reproduced in this issue initial. As I can see, it fails because plugin can't convert API level requested into SDK target to operate with.
When I have modified API axis to "15 19 21 23 androis-24" - plugin have successfully created AVDs.
IMHO it is rather strange to write "API level" for some versions and "SDK target" for other... Please fix it.