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 )