why can't I use some web sites such as United airlines
There are websites that I cannot use such as united airlines
iMac 24″, macOS 13.2
Read If Safari doesn't open a page or work as expected on Mac - Apple Support.
The usual suspects are VPNs, non-Apple "anti-virus" / "security" nonsense, that sort of thing. That Support document is comprehensive; it addresses those potential causes and more.