I Was Diagnosed With Narcolepsy Last Wednesday - My Sleep

I was diagnosed with narcolepsy last Wednesday - My sleep physician ordered me Vyvanse 20mg - my insurance company had denied the medication claim stating that the doctor has to try amphetamine/dextramphetamine along with 5 other medications, all of which is not the generic of Vyvanse. It Is it legal for a pharmacy (express scripts) to be able to force my doctor to try other meds when he clearly wanted me to take Vyvanse?