Just had this issue today. Submitted a ticket.
I thought maybe because my washer fluid was out, it wouldn't enable because it couldn't clean the cameras if needed. I topped up the washer fluid and it is still happening. Just a message saying "cruise control not available". There are no other active alerts like camera blinded, reduced visibility, etc.
Tried the two button reset and that didn't help either. It's an overcast day so no sunlight washing out the cameras. I'd see a message of that if it was happening.
I haven't attempted to use AP/CC since the last software update until today, so maybe the latest one broke it. Sigh. Always two steps forward, one step back with software. Not just with Tesla, but in general.
I wish Tesla would implement a dumb cruise option with no crash detection or other safeties. Software will never be perfect, it's only as good as the humans programming it, which are never perfect. Even if it was a menu item you had to activate at the start of the drive and kept a persistent icon reminding you that the vehicle won't brake automatically. Similar to the message when you hold the accelerator down a bit during CC.