The filename follows Jekyll's specific filename requirements.

vvaaaalu/github-pages-with-jekyll/_posts/2020-05-16-title.md 

I can’t check my post. 

please help me…

Hey there, @vvaaaalu. I get a 404 error when trying to visit your repository. If you’d like to make the repository public, I’d be more than happy to take a look. At first glance, the title you described should work!