Why I can’t update my iPhone 7 ?

you've really provided no information that would help us help you...

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..

Please read through the user tip below for possible causes.

https://discussions.apple.com/docs/DOC-250003031