Hey everyone, I need help with a Leaflet

For a start, I do not understand programming and HTML. I need an interactive map, for a play-by-post RPG and not finding a suitable solution, I came across a Leaflet and this tutorial (https://www.youtube.com/playlist?list=PLGB6RkFB7ZmPCPfuoZZI_r_7w0wSok79M ).

And already at the second step of the tutorial I ran into the problem that my result does not coincide with the result on the video, namely, I see only an empty page.

Repository with my files used: https://github.com/Nayderi/nayderi.github.io