add_theme_support( 'post-thumbnails' ) not working?

Question

WordPress

Hey!
I add the code add_theme_support( 'post-thumbnails' ); to functions.php, but the window for inserting a thumbnail does not appear in the panel, tell me what's the matter?
On an English-language site, I found a kind of answer https://teamtreehouse.com/community/-i-added-addth... but I could not understand what needs to be done to make it work.

Answer 1

I solved the issue, the names of the funсtions.php file were written using Russian C :)