Note the limitations from the manual:
It may take up to 15 minutes to activate once you leave vehicle. Your target temp might be set higher. You may have "no A/C" set (in which case it runs only the fan, which may not be sufficient to cool the cabin). It only activates for the first 12 hours parked and also when battery SOC is above 20%. Read the manual for more details.
Model 3 Owner's Manual | Tesla
The people above are describing a different scenario of it being grayed out, not that it can be activated and doesn't seem to do much. Also the standard tips of rebooting car, updating app, toggling the desired setting on and off to see if it changes anything also applies.