Why I can’t install new software update ??

You must have a stable Wi-Fi connection.

Charge the battery to 50% or higher.

Turn off Battery saving mode.

Don't use a Hotspot.

Don't use VPN.

And make sure you have enough free space.

Note:

As you are too far behind with updating your iPhone

you probably have to connect to a PC with the latest version of iTunes installed.

Its telling you why in your screenshot:

You will also need to be connected to Wi-Fi