The MSE could be Inf for several reasons. My guess is that you have a poorly posed fitting problem. Did you see any warning messages during the fit? That might give you additional clues.
Also, you can find the MATLAB code that does this calculation in function calcCovBeta of class SimBiology.fit.internal.FitObject, but let me summarize the calculations here. MSE is the sum of the sequares of the residuals divided by the degrees of freedom. If you try to estimate 2 parameters with 2 observations, your degrees of freedom will be 0, and the MSE will be Inf.
If you still don't understand why the MSE is Inf after reading these details, then I suggest sharing reproduction steps. Then, we can step through the calculations in the debugger and see exactly where and why the Inf value occurs.
