I cannot install the latest update idk why.

What happens when you try?

If you have an add-on “security” app or an add-on VPN app, remove that, restart, and try again.

Failing that, use a Mac or Windows computer to update.

You must be connected to Wi-Fi, or to 5G with a carrier and phone that supports 5G access for updates, or must use a computer.

Using a computer is described in the update article:

Update your iPhone, iPad, or iPod touch - Apple Support