Hey there!
So it looks like from the image that you do not have any baked textures slotted into your Material Section. When using the Mesh ID to Mask option, that slot will require a baked map to create the mask from. If you have already baked that texture, just make sure to add it to that to the appropriate slot shown below! It also looks like you do not have any other baked textures input as well, I would recommend you take a look and add any that you have baked, or take a look at baking some out if you haven't already. :)
If you haven't baked that map yet, you can select that as one of the baking options in the baking dialog. It "should" automatically be added to that Material Section, however if it's not then you may need to manually add it.
