The variable matrix multiply its own conjugate transpose

I used CVX for the first time, and there was a problem . I hope you can help me.

In addition to V_a and V_b, the rest are constant. V_a and V_b are matrix variable of 2*2, we want to maximize logdet(\sigma_a), but CVX errors in running V_a multiplied by V_a’.

Error using * (line 126)
Disciplined convex programming error:
Only scalar quadratic forms can be specified in CVX