Why cant I block an email on iphone 7?

Welcome to the Apple Support Communities!

I understand you want to block an email sender in the Mail app on your iPhone 7. As a general rule, you would block emails from your email service provider's website so that these messages never make it to your iPhone. If you subscribed to an email list, there should be an option to unsubscribe. Here is some general information about using junk mail and blocking contacts on an iPhone. If you have a computer, adding email rules may help, but the best solution is at the source (your email provider's website).
Identify and filter junk mail in iCloud
Block phone numbers and contacts on your iPhone, iPad, or iPod touch​
If the individual is in your Contacts, you can add them to your Blocked list.
This mainly applies to phone calls, messages or FaceTime, but the email address field is included in the contact card.
Tap Settings > Messages > Blocked and tap Add New at the bottom of the list.
Take care.