Validation error

I try to validate my themes in html 4.0 transitional via w3schools, and I keep getting this error for all my themes:

3: <b>Fatal error</b>: Call to undefined function: get_header() in <b>…../users/alias/wordpress/wp-content/themes/hmtwice/index.php</b> on line <b>2</b>

I don’t know how to fix this :/

Here’s the validation page link–
https://validator.w3.org/check?uri=http%3A%2F%2Fwww.unknown-entity.org%2Fusers%2Falias%2Fwordpress%2Fwp-content%2Fthemes%2Fhmtwice%2F&charset=%28detect+automatically%29&doctype=HTML+4.01+Transitional&ss=1