Maxima cannot do any mathematical operations (differentiation, limits,
integrals, etc.) on a named function defined like this.
It is also very ignorant about non-analytic functions like signum, abs, etc.
To calculate the limit, you'll have to extract the relevant formula,
1-exp(-(b*x)^a), and work on that. Of course, in this example, that's
trivial -- I suppose you have some more elaborate cases in reality.
-s