You have to use the Non-Linear Curve Fitting Analysis Object instead of the Approximation function because the Weibull function is a non-linear function.
With the Non-Linear Curve Fitting analysis object, you can converge a model function dependent on an independent variable and several parameters toward a given data set. A number of pre-defined model functions are available to do this. Alternatively, you can define particular models.
E.g. you can use the predefined Weibull model (variante 1). Certain parameters of the selected model can be declared as fixed.
Non-Linear Curve Fitting Analysis Object
Non-Linear Curve Fitting Tutorial