Hey Alfonso, 
Thank you so much for your speedy reply! I did notice an error in Matlab (I was running the analysis while I was out of the lab so I am not sure exactly when the error appeared but I would guess it was before the error using cat).

This error was: Warning overdetermined model (no degrees of freedom for Subject 7 session 2). Please consider reducing the number, dimension of covariate, order or the confounds, or disregarding this subject/session. 

I also got the error for Subject 8 session 2. Could this be related to the Error using cat? 

Best, 
Leah