My iPhone 6s Plus can’t update iOS 15 why?

You need about 6 GB device storage, see this Apple articles below

If you need more space for an update on your iPhone, iPad, or iPod touch

https://support.apple.com/en-us/HT203097

How to check the storage on your iPhone, iPad and iPod touch

https://support.apple.com/en-gb/HT201656

Clear Other storage on your iPhone or iPad

https://support.apple.com/en-us/HT211686

What's the difference between device storage and iCloud storage?

https://support.apple.com/en-us/HT206504

Check the range of the mobile network, see this Apple article: If you see No Service or Searching on your iPhone or iPad

https://support.apple.com/en-us/HT201415

Check also the Wi-Fi network, see this Apple article: If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

https://support.apple.com/en-us/HT204051

See also the following Apple articles:

Update your iPhone, iPad, or iPod touch

https://support.apple.com/en-in/HT204204

If your iPhone, iPad or iPod touch won’t update

https://support.apple.com/en-us/HT201435

What is the existing version of iOS on the iPhone?