If you are having any errors or other issues, please read this self-debugging guide first- and please do not report issues anywhere besides this discussion, or i might delete your comment. also, please do not follow the fixes below unless you actually have their corresponding problems, otherwise this may cause additional issues for you.

with that out of the way, here are potential fixes for common problems (not guarunteed to work but worth a shot):

This map is extremely detailed and almost every sky-light is a dynamic shadow (via env_projectedtexture.) as such, your machine should probably have decent specs. don't expect this to work well on a laptop. That all said, if your machine has good specs, try the following:
1. uninstall or disable Extended Spawnmenu, and Map Retexturizer. these may cause problems for certain people on this map.
2. install the "doors" addon (the one that adds worldportals)- i already included it's files, but having the real thing might be helpful, and im planning on requiring it in a future update.

Please report any errors (the actual errors, dont just tell me "there are errors"). I need to know in order to figure out how to fix it, if i even can.

As above, make sure your pc can actually run dynamic lights. a good test is to spawn several lamps (not lights) and see if the shadows work, on gm_flatgrass. if they dont, then your computer wont be able to use the shadows on this map properly.
in which case, paste this in the game's command line in advanced steam properties:

Please send me screenshots of where you specifically are seeing these so i can fix it. Please do not just tell me "there are errors" without describing the area where they are at in detail, as i will not be able to fix it if i cannot determine which assets specifically went missing, and therfor wont even bother.

type the following in the console: garry's mod likes to randomly screw up cubemap reflections for random people, this isnt something i can fix, so please dont report it.

i've done my best to optimize the map, and continue to do so.
if it runs poorly, your computer probably cant run it. I apologize for the inconvenience.