A centerline section can contain only 1 path and as many direction lines as you need. Remove any other element from the section then retry. (problem 10)
There is more than one path in this centerline section. Try to create multiple sections using a single path for every section. (problem 9)
A part of this section seems unconnected. Divide it into smaller sections or increase the density. (problem 8)
In order to calculate you must insert a direction line. (problem 7)
The direction lines are placed such that they intersect each other or form too large an angle. (problem 6)
Direction lines as drawn create an ambiguous area. Try adding new borders (problem 5).
At least one path is needed to apply this effect. Add one or more and retry. (problem 4).
The direction line is too short. Lengthen it and retry. (problem 3).
The direction line does not intersect any closed paths. Relocate it and retry. (problem 2).
It is not possible to apply the effect because there are no closed paths in this section. (problem 1)
