Sorry, Mark, I missed that you are trying to build a mesh from lidar points and not from photos. Yes, mesh form depth maps or mesh from dense cloud requires aligned set of photo to be available as the source options in the mesh dialog.
If your las is classified you might use ground class to get a DTM if it's not then you can try the ground classification tool and later do the same. Normally you cannot expect to have a nice mesh from forestry lidar data because the points are everywhere into the canopy structure and form a scattered "fog" inside, making it hard for the program to estimate point normals and thus deliver a detailed and accurate mesh. If you are lucky enough and have enough points hitting the ground, you can at least create a good terrain model... but yes, all are pro features.
José Martínez
Accupixel & Geobit
Metashape training