Why I can’t upgrade my software I’m using a IPhone 6 Plus help me

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.

You can only update to iOS 12.5.6, the iPhone 6 does not have sufficient RAM to run iOS 13 or later.