Meta object development in Blender is very limited . You cannot attach an armature (or any modifiers for that matter) directly to a set of meta balls like you can with mesh objects .
The only way you could attach it to an armature would be to directly parenting the meta objects that compose the head to the head bone, the objects that compose the torso to to the torso, etc. while in Pose Mode for the armature . So after tabbing in to Pose Mode with the armature selected you would shift select the all the parts then shift select the armature bone they belong to last and use the Ctrl-P hotkey to parent them to the bone .
But you have a very fundamental problem … you started modeling the hippo from the side view and backwards … It would be an easy thing to select all the objects and rotate the entire thing 90 degrees so that it is facing the front (you’d want this to use the mirror function of the armature), but one or more of your objects has it’s center located several BUs from others and makes this almost impossible … and this also might give you weird results when animating with the armature because of this offset . Most likely you moved the meta ball(s) to its current location while in Edit Mode …
You could convert the entire thing into a mesh and then delete one half and use the mirror modifier to reconstruct the mesh then add an armature the conventional way (after making it face the right direction and applying the rotation with Ctrl-A) …
Rigging anything requires that you understand and position your objects properly in 3D with no unwanted rotations or object centers “misplaced” . You need to be organized and understand the basics from the very beginning to get a successful result …