Can't enable collision on a Skeletal Mesh

Hi,
I use a character downloaded on Mixamo in a Pawn Blueprint. All works good but when I want to add impulse to make the pawn jump, I need to activate “Simulate physics” and add collisions. And there is the problem : enabling collisions make the character floating in the air (like a ragdoll astronaut !). Can you please tell me what’s wrong ? Mixamo provides the physic asset for the character and it is well applied on the skeletal mesh. Thanks !

Images with collisions enabled :