I first interpreted "lower" as "lower on the hierarchy" as well, but it seems it just needs to be instanced/read before the SkeletonIK node. I moved my target node to be literally further up, but still having the same relative path ("../../../Target") and the error message disappeared.
Before:
Spatial
Skeleton
SkeletonIK
TargetNode <-
After:
TargetNode <-
Spatial
Skeleton
SkeletonIK