Hi, i have a question, i did an animation from blender and when i export the fbx document and try to import this with UE4 this error apears :s help me please, “how can i change the multiple bone to single root bone”?
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First, unparent your mesh to the skeleton (or this may cause crashing later when importing) alt+p, clear parent.
Now, go and create a root bone if you do not already have one you want to use as root.
Join all your bones together into one armature by selecting them all in object mode and pressing ctrl+j.
Select the armature and go to (usually tab)edit mode, select all the bones that you ARE NOT your root bone, then finally select the root bone and press ctrl+p (make parent), and select keep offset, (assuming you dont want to move your other bones onto the root bone).
Export the armature as fbx as you where and it should work now.
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I have the some error 10 minets ago:D see exapmle:
Firlsty i have error, i have this bone structure:
To fixed it i chane it like that(with some method create bone and root all to it(ctrl p → keep offset/alt p to clear), also you can just add parent to “Hips” on my exapmle and 4 another bones to root bone, and all)
If also dont understand see this posts Blender import error, multiple root bones. - Animation - Unreal Engine Forums
And also i create tutorial - YouTube )
I had the exact same problem. I made a skeleton mesh in blender with armature. When I exported and imported to UE4, I got a multiple root error. I went back in blender and noticed that my IK and pole targets were not under the Root hierarchy. I selected the IK and pole targets, then selected the root bone, Ctrl-p offset. This made the root bone the parent of IK and Pole-targets. Then I went and unchecked inherit location, rotation, transformation, and connected on IKs and Pole Targets, so it didn’t mess everything up. Re-exported to UE4 and works just fine.
I did all this in edit mode and didn’t have to clear parents or join anything. When you make the IK and pole targets, you have to clear the parents on them to work correctly, but after that you have to re-parent them to the root as I stated above, or UE4 will think its another Root Bone.
Thanks for your help, , you got me on the right track. 
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This is a solution that worked for me.
262917-
First collapse every parent until it looks like this
Then create a new bone, call it ROOT and put it anywhere in model space (I put mine right above the character head but it doesnt matter) then select every parent bone (Torso, Leg Control R, etc) selecting the ROOT bone last and ctrl+p in edit mode and select keep offset
SOLVED: It should now look something like this. Notice the root and everything under it. Export as FBX and there should be no problems, at least in my case.
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Yeah… So i came across this very same issue and tried all those methods! And they WORKED! but you know what worked the easiest? Change the naming convention of the ‘Armature’ to Root… I kid you not… it was that easy. i guess UE doesn’t like Armatures
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This works for me
---- UnrealEngine 5.20
---- Blender 3.6
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You da’ real MVP
The 3D modeling and animation of bone hierarchies is limited. Despite the fact that creativity knows no bounds, certain software like Blender restricts us to a single root bone. Because of this, complex structures and creatures with multiple bone starting points may require creative workarounds, but this is a challenge that pushes artists and animators to think outside the box, leading to innovative solutions and pushing the boundaries of what’s possible with 3D graphics in the digital world.
I had this same problem today and i found a solution that has worked for me.
The solution to the problem of Unreal saying you have multiple root parts was that the name of the Armature was somehow affecting it so by changing the name you will be able to import the .FBX file from blender to Unreal without any problems.
Here is the video link i found for the solution → https://www.youtube.com/watch?v=Wnq4Lbr0PC4