The error message description is not as clear as perhaps it could be. This error message is not due to dimensional incompatibility (number of rows and columns), but does indicate a violation of CVX’s DCP rules. In particular, 2*R*C*E.*PB violates the rule prohibiting multiplication of two different CVX variables or expressions, in this case, E and PB.
If he constraint E-sqrt(E.^2-2*R*C*E.*PB)<=Imax*R*sqrt(2*C*E*nb) is convex, it will need to be reformulated. Have you proven it is convex?
Until you show otherwise, I will mark this problem as non-convex.