What is the problom with my phone?

-Make sure iPhone have Enough Storage for update at least 6GB.

-fast Wifi Network with Internet access (no cellular data, unless you use a 5G network, no personal hotspot on your device, no VPN connections are admitted, no Low Power Mode enabled

-you cannot update over cellular data or a cellular hotspot

Here --> Update your iPhone, iPad or iPod touch

Here --> If your iPhone, iPad, or iPod touch won't update - Apple Support

Maybe your storage is full thus not allowing you to perform a software update?