To let your ragdoll work, you’ll need first to modify the phisics asset of your Skeletal Mesh.

Open your phisics asset and modify it as follows:

Set the phisics type of the root as Kinematic and set his collision response to Disabled.

If your phisics asset does not have a Root shape, click on the settings button on top, select, ShowAllBones and then you should see the whole skeleton in the Skeleton Three window.

Now add a sphere shape to the root:

Then add a constraint from the root to the pelvis:

Then, as described before, s et the phisics type of the root as Kinematic and set his collision response to Disabled.