Why I am unable to update my iPhone 6 Plus to new iOS version?

You cannot update your iOS version any farther because the hardware on your phone won't support a newer iOS version.

You cannot download the new version of messenger because Meta has chosen to only support iOS 15. Each developer chooses which versions of iOS they would like to support and can drop support for older versions at any time.

The iPhone 6 Plus was discontinued 8+ years ago, so you are unfortunately about that far behind the curve.

If you want to run iOS 15, you will need an iPhone 7

If you want to run iOS 16, you will need an iPhone 8 or iPhone X

If you want to run iOS 17.....and soon to be introduced iOS 18, you will need and iPhone XR or higher model

My recommendation would be to go for as high a model number as you can, so you won't have to be worried about obsolete iOS versions for another 7-8 years.

Apple released the iPhone 6 Plus about ten years ago. When it came out, it was running iOS 8. Apple supported it with updates all the way through iOS 12, but the iPhone 6 Plus is too outdated to update any further than that. The iPhone 12 will never be able to run iOS 13 or later.

MacTracker's database provides some clues as to just how outdated your iPhone's hardware is.

• Your iPhone 6 Plus has 2 CPU cores and only 1 GB of RAM. It also lacks a Neural Engine.
• Only phones with at least 2 GB of RAM are compatible with iOS 15. Many of these iPhones had Neural Engines, but the earliest phones capable of running iOS 15 – the iPhone 6s, 6s Plus, SE (1st gen), 7, and 7 Plus – did not. Those early phones also had only 2 or 4 CPU cores. These are the ones that cannot update to iOS 16 or later.
• Only phones with 6 CPU cores, a Neural Engine, and at least 2 GB of RAM are compatible with iOS 16. The first Neural Engine was in the Apple A11 Bionic processor in the iPhone 8 and 8 Plus.
• Only phones with 6 CPU cores, a Neural Engine, and at least 3 GB of RAM are compatible with iOS 17. Only one or two of the iPhones that are compatible with iOS 17 have as little as 3 GB of RAM. Most have 4 to 8 GB.

Or, to put it another way:

• iPhone 6 Plus (2014). : 2 CPU cores, 1 GB of RAM, no Neural Engine, 16 – 128 GB of flash storage, runs iOS 8 through iOS 12.
• iPhone 15 Plus (2023): 6 CPU cores, 6 GB of RAM, 16-core Neural Engine, 128 – 512 GB of flash storage, has support for 5G cellular service, runs iOS 17, will run iOS 18, and will almost certainly run versions beyond that.

Nine years of advances in technology makes a difference.

Apple released the iPhone 6 Plus about ten years ago. When it came out, it was running iOS 8. Apple supported it with updates all the way through iOS 12, but the iPhone 6 Plus is too outdated to update any further than that. The iPhone 12 will never be able to run iOS 13 or later.

Typo. I meant to say iPhone 6 Plus.