Why cant I use PHP echo in an HTML document?

Today i am using Php ECHO IN HTML DOCUMENT AND PLEASE EXPLAIN ME Why cant I use PHP echo in an HTML document?

PHP is a server technology (mostly). There’s a PHP program running on the server, and pages coded in PHP work with it. The program interpret the code and serve HTML CSS JS. Often works with a MySQL database.

Webflow doesn’t use PHP at all, nor MySQL. That’s why you can’t use PHP.