I used to have this probelm, first of all, make sure bsp in
base/maps/
In-game, I dont kjnow why this works, but I get the same error as you, hoever for me if I go
/sv_pure 0
/map arachnid2
wait till its loaded up then type
/devmap roomy
It works!
For me, try it and see if it works for you.