That’s probably because you copied it from the Quixel Bridge library, which will have updated Metahuman 5 files, so the wrong ones. You need to copy the mocap folder from an (older) UE project that has a Metahuman 4 in it and copy over those files.
So your older project with Metahuman 4 will be: [project name]\Content\MetaHumans\Common\Common\Mocap
And it’s that folder you have to copy over to your new project that already has a Metahuman 5 imported. You will need to do this for every new project and every new Metahuman you import.