I can't update my iOS 6s there is an error written an error accorred to install iOS 15.5 what I do

Check iPhone type & iOS version -> Go to: Settings -> General -> About.

If it is an iPhone 6 -> iOS 12.5.5 is the last security updates for iPhone 6, -> released 23 september 2021.

If it is an iPhone 6S you can update the iPhone to iOS 15, see this Apple article:

iPhone models compatible with iOS 15

iPhone models compatible with iOS 15.4 – Apple Support (UK)

Go to: Settings -> General -> Software Update.

You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

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

You need about 7 GB device storage and good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.

How much memory is in your 6s? My 128GB devices run 15.5 just fine. If you have memory constraints, you may need to update your phone via USB connection to an iTunes computer. Could that be your problem?

See this --> If you see an error when you update or restore your iPhone ...

Or this --> iOS update and restore errors - Apple Support