Learn Hardcore Russian: Этот сериал очень популярный.

Russian normally drops the present‑tense form of быть (to be) in sentences like this.
So instead of saying Этот сериал есть очень популярный, Russians simply say Этот сериал очень популярный.

Literally it looks like “This series very popular”, but it is understood as “This series is very popular.”
If you add есть here, it sounds either archaic, overly emphatic, or just wrong in modern everyday Russian.

Why is it этот сериал, and not это сериал or эта сериал?

The noun сериал is masculine, so the demonstrative “this” must also be masculine:

этот – masculine singular (this)
эта – feminine singular
это – neuter singular
эти – plural

Because сериал is masculine, you must say этот сериал.
Это сериал usually means “This is a series / TV show” (identifying what something is), not “this series.”

What gender, number, and case is сериал, and how does that affect the other words?