Multible roots are found in the bone hierachy

So I downloaded an FPS arms model with weapon and animations from Sketchfab and tried to import it into my UE4 file. But everytime I try it i get an error message that it failed to import and in the logs it tells me this: “Multible roots are found in the bone hierachy. We only support single root bone.” I tried importing it into blender and tried to fix the issue but but i cant find the solution. This is the model I downloaded: fps AK animated - Download Free 3D model by DJMaesen (@bumstrum) [b0b1bae] - Sketchfab
If anyone could tell me what i am supposed to do I would be very grateful