how do ı connect html files each other with jinja?

Question

I'm trying to create a website, and for that I need to separate my codes. Here is my files structure:

• templates(folder):
  • index.html
  • header.html

I tried this code in my index.html file:

{% include "header.html" %}

but it didn't connect each other and wrote it in my website {% include "header.html" %} as a string.

where is my mistake, what can ı do? if you help me ı would very happy