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 select the newly created constraint and set all the three angular limites to Fre
As described before, set the phisics type of the root as Kinematic and set his collision response to Disabled. e.