make sure its enabled in gmod. makesure you also have tf2. makesure you restarted gmod after enabling css

CS:S was already enabled

and the other 2 things i said?

I think I might try this from the "help" you have given:
1. Enable everything in GMOD
2. Close GMOD
3. Run CS:S
4. Close CS:S
5. Run TF2
6. Close TF2
7. Run GMOD
8. Try again

maybe INSTALL CS:S? try to see if it needs to be installed,if so when you click on it in your library,the "play" button is replaced with "install" see if it works,if not run gmod then click on the little controller at the bottom right corner of your screen,a list of source engine games you own will pop up above the controller,if not done click on the little box that reads CS:S or Counter Strike:source.doing so will mount it on to gmod.if you've done any of the following already,then idk what else to do,maybe re-install it. hoped this helped ;)