

If you ask Prism to use either the F test or the AICc method to compare two models, it will report "Can't calculate" if the two models have the same number of parameters. This means it can't calculate the comparison between fits, not that it can't fit the two models to the data.
Both these methods to compare models only make sense when the models being compared have different numbers of parameters, and so have different numbers of degrees of freedom. These methods are alternative methods of dealing with the tradeoff between how well the model fits and how complicated it is (assessed by the number of parameters).
If you want to compare two models with the same number of parameters, there is no need to use either the F test or AIC. Simply choose the model that fits the data the best with the smallest sumofsquares.