Ensure that you're subscribed to the content if you've restarted your game and it's still an error then press escape go to addons - subscribed and make sure the content is enabled.
If that's all done then make sure Half life 2 episode 1&2 are installed and mounted in addition to that also Counter strike source.
Once you installed the games, load the game up then exit out of them and go onto Garry's mod, from there go to the bottom right, there is a joystick press the joystick and make sure the games are mounted.
Content is here for each server;
https://nebulous.cloud/threads/server-content-ips.1072/