1999 Nissan Maxima Gas Tank Will Not Take Gas

When I try to fill my 1999 Nissan Maxima tank with gas at gas station, it will not fill on its own. The pumping stops. The attendant has to keep pumping. When they try to pump, it keeps shutting off. Attendant stated he thinks I have air in the gas tank when he took the cap off. I am also getting a Check Engine Light Error Code PO450.

Please advise