My guess is that the mesh name that you are passing as the first parameter to importMeshAsync isn’t actually the name of any mesh in the obj. It looks like the obj’s mesh name is Shark_01SG1_Untitled and not shark. If you just want to load everything from the obj, pass in empty string or null as the first parameter for importMeshAsync.
If this isn’t the issue, can you put this into a playground?