I couldn’t update my iPhone 6s

Ok, see if the following article can help:

• If your iPhone, iPad, or iPod touch won't update - Apple Support

If any of the following are true, your phone may not be able to update:

• The phone is running an iOS beta. If so, then you will need to remove the beta profile first.
• The phone does not have enough free space. Typically it should have at least 5GBs available to update.
• The phone is configured for either or both Low Power Mode or Personal Hotspot.
• The phone is not connected to power and it has less than 50% charge left.
• The phone is not connected via Wi-Fi to the Internet. It won't update on a cellular-only connection.
• The phone is connected via a VPN.

When all else fails, doing a complete install from a computer may work better. It's also preferable because the computer will back up the data on the computer before starting the installation. It also requires less free space because it uses the computer's storage for a lot of the unpacking.

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

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

sargis128 wrote:

If my understanding is true I can’t update with windows operation system and Need macOS?

You can if you have iTunes installed. Check the second link above. This one:

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

You would need iTunes for Windows.

Download and use iTunes for Windows - Apple Support

There might be issues if you have an older version of Windows. Good luck.

... and what did this notification say?

Ref: If you see an error when you update or restore your iPhone, iPad, or iPod - Apple Support