Is it safe to take creatine and multivitamin supplements while on accutane? I am an 18 year old male wanting to gain lean muscle mass.

Safe but no benefit. The data on creatine remains controversial but at least it's safe. Same w/regards to multivitamins. Luckily, no interactions w/Accutane (isotretinoin) either. Best way to gain muscle, besides exercising regularly, is to eat healthy & get enough rest. Make sure you get enough protein 1-1.5g/lb body weight daily.
Be cautious. accutane is vitamin A, and it isn't a safe drug, must be taken under strict supervision, lots of side effects and possible drug-drug interaction, ask your doctor if you want to add any other medication, and restrict accutane usage for the least duration possible, even if you are a male, take care.