"I was not sleeping."
Translation:Cha robh mi a' cadal.
Yes, definitely! Actually I believe cha robh mi nam chadal would be much more natural, and also should be accepted here.
I think that the course doesn’t teach (at the moment at least) such construction like tha mi nam chadal I am sleeping, tha e na shuidhe he is sitting, bha i na seasamh she was standing, etc. and that’s the only reason that they use the less natural a’ cadal, a’ suidhe, etc. in the course at the moment.