Max Take Off Mass is the maximum weight that the drone can be before it will be able to safely take off. DJI have published them in the past but not for this one. You need to know this as you could be breaking the law if you fly outside of the manufactures specs. The absolute Max take off weight will differ from the weight you can safely take off with.
It seems you are wanting to use your DJI Drone as a Cinelifter? You could be breaking the law inside the specs really, I suppose. I don't find the spec. anywhere either.
No I do not want to use it as a cine lifter at all, I am taking the GVC course in the UK and knowing the MTOM of your drone is something that you are expected to know. One for knowing the law and that you are not breaking it by flying the drone outside of the manufacturers specification. The same as you are expected to know the maximum wind speed that it will safely fly in.