Why I can’t update my iPhone?

Go to Settings > General > About

What does it say for iPhone Model and Software Version?

You need an iPhone 6s/6s Plus or newer in order to run iOS 14/15..

Supported iPhone models - Apple Support

If your iPhone is really an iPhone 6 like your details show.. and you are expecting to upgrade beyond iOS 12, you cannot

have you read thru this support article for updating?

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

need more storage for update?

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

issues?

Updating iOS: common issues and solutions - Apple Community

video..

Because it is not compatible with iOS 13 or higher.