Huge Bill for labs that doc said would be covered

I had an appointment with a dermatologist last month at Sutter health. They’d sent me for some tests within Sutter Health. At that point the doctor had (verbally) informed me that tests would all be covered by my insurance.

Now, a month later I have received a huge bill of 400$ for those labs, with some of those tests not being covered by my insurance at all.

What can I do to understand and fight these charges? I would appreciate any sources I could use to understand more..

Thanks in advance !