From the info, you gave can't tell where's the problem. Does the error happen all the time? If you could provide me with your blender file and the steps on how to get this error, I could fix this error.
Viewing post in Unreal Engine 4 Vehicle Rigging Addon for Blender comments
My fault it's been a while and I haven't touched it since. I got it working in the end by just doing it from scratch.
I apologies I cannot provide what you request atm :/
Best I can give you is try renaming bones.
The reason it didn't work first time is because I didn't follow the video step by step, I went slightly off course when I tried renaming things.