Why I can’t download iOS 13.4 on my iPhone?

You don't have a 6S plus, you have a 6 plus.

An iPhone 6/6plus can't be updated beyond 12.5.6, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

What is the existing version of iOS?

iOS 13.4 is long out of date and not available. To see if your iPhone is qualified for a current update......

Tap Settings

Tap General

Tap Software Update

What message do you see?

Then you will need to purchase a newer phone capable of running a newer version of iOS.