We can't really say exactly what the issue is without all the inputs to your frestimate call- your model (mdl), your io points (io), and your input signal (in). This error message is thrown if the sample time of the signal corresponding to io(1) is not compatabile with the sample time of in. The sample times are compabile if they are both discrete with the same sample time, or if the signal corresponding to io(1) is either continuous or fixed in minor time step.
If you cannot resolve the issue, you should make a help ticket with techincal support.
