The geometry as well as the boundary conditions need to be symmetrical
The part must be located in the coordinate origin
Since the forces and moments always affect an object/volume they must be adjusted. If only considering half the part, the forces and moments need to be divided in half as well.
The symmtery constraint might not show exactly symmetrical designs while choosing the calculation type “estimate”. This is because of the low resolution level but will not occur once the calculation type is set to “optimizing” and the optimization runs through.
Further information here.