Complex Data type not accepted in Simulink
The data type of the signal must be fixed in Simulink. Since you use a square root of a real signal, the output will be real if the data is positive, imaginary if negative. Simulink cannot allow that.
The workaround is to use force your real signal to be a complex signal by setting its imaginary part to 0. Then feed the signal to the square root block. The output will always be complex.