As shown in the slide, first slide says use simulation for complex system(point no.1) and second slide says don't use simulation for complex system(point no. last). What is going on here? Should I use simulation for complex system? Or should not I? And why should not I use simulation when managers have unrealistic expectations?

 

Without any context or examples it is impossible to know what the slide author regards as 'complex'.
In general, the more complex a system is the longer a simulation takes. Breaking a 'complex' system down into functional sections is usually possible and simulating each section separately speeds things up.
No component model is perfect, so the overall simulation of a complex system with many components may have a greater error than the simulation of a system with fewer components.