Why I can’t update my phone

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.4

• If you have an iPhone 6S or newer, you can update to 14.7.1

Update your iPhone, iPad, or iPod touch -

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

Do you really have an 6s Plus or just a 6 Plus? If the latter you can only go up to iOS 12.5.4.

If the former, did you follow the steps here -> Update your iPhone, iPad, or iPod touch - Apple Support

Update my phone

It's already been posted twice in here how to do so.