WP "More" Content Block not Working

Hello,

I am writing a very long blog post. So I want to add a "read more" and "read less" text link to some content elements on the same post. Unfortunately, the Wordpress built-in solution "More" doesn't work on my page?! It just doesn't hide any text, and the "read more" link doesn't appear either.

I also installed the "Read more without refresh" plugin, but that doesn't work either. The plugin is completely without any function...

Does anyone have a solution for me? A JS & CSS code would also be nice, if that helps me to add a "read more" and "read less" text link on my page.