I created a map, everything is ready, but there's a "small" problem - the map doesn't appear in the list of other maps, and I can't test it. I placed the map in HMMV\Maps.

If the map size is not the issue, try going to map properties (by pressing M). In the Player Properties tab, specify everything for all players on the map. + in teams, if needed. Maybe there's something else, i.e., clearly "tell" the editor what players there are, what their starting cities are, etc. Save. Load the game. If all this has already been done, the map may be corrupted or the path to it is incorrect.

Added 7 minutes later

How can I place city buildings as objects in the editor? I only found their models, but they are invisible for some reason...

I did this. I placed several city building models in one point, and that's how I created a city. I placed the coordinates approximately at -80, -80, -43. However, the video doesn't play on such a map, and the map itself is buggy. Probably because of these coordinates