I still can not get through chapter 12 as Lucas is completely missing. Even some parts of his voice are now gone. I have made it to the point of the lion's mouth but can't get past that point. I have sent a save game but have not had any response. Really frustrating as the game is super fun but now I am totally stuck. This is on a Ps4
This is a pretty common bug. Look around on this forum or on Steam forum and you'll find lots of people having problems with Lucas in this area.
What happens is that if you die to the rats in the area below the floor of the symbols (the room with the lions head), Lucas will bug out when you re spawn. Sometimes he disappears, sometimes he gets stuck inside a wall - or sometimes he spawns with you under the floor and is unable to get upstairs again.
In any of these cases it is impossible to complete the task and interaction with the lions mouth is not possible.
As of now a restart for the beginning of chapter 12 is the only solution I'm aware of.