Fidelis denied claim

Went to a follow up dermatology visit. First visit was covered no problem and all that. My insurance benefits are medicaid so everything seems to be fine if the place is in network. I had no issues at all with my first visit so I assume they're in network.

I got an email saying my claim was denied due to "Failure to obtain authorization prior to rendering services"

Do I need to call the insurance company and hash this out? Or is it on the provider? I got the email and it was addressed to me.

Edit:
Unsure if the fidelis find a doctor function is correct, but I searched the place I went to and it didn't pop up on the list of their doctors. But the first visit went smoothly and without hassle. No copay or anything.