Your cell commands are creating cell arrays of empty matrices. Therefore, all your subsequent operations create empty matrices. Therefore sm is an empty matrix as of the maximize(sm) statement, hence the error message. You would better see what is going on if you removed the semicolons at the end of statements.
I don’t know what you are trying to do, but perhaps it would be easier using multidimensional variables in CVX. .See mcg’s answer at Cvx sdp mode and cell arrays .
For example
variable joeblow(4,6,5)
Once you get the problem accepted by CVX, and given your use of log, I suggest you follow the advice in CVXQUAD: How to use CVXQUAD's Pade Approximant instead of CVX's unreliable Successive Approximation for GP mode, log, exp, entr, rel_entr, kl_div, log_det, det_rootn, exponential cone. CVXQUAD's Quantum (Matrix) Entropy & Matrix Log related functions .