SQL foreign keys have a direction (from the PKey to the FKey), so you can have more than one path from one field to another in the SQL foreign key structure.
By contrast, Qlik models are associative databases and an associative database has no concept of direction of an association. So multiple paths between two fields cause possible ambiguity and are therefore disallowed. Usually this problem arises when the data model design has not been properly developed.
These links apply to QV models, but the same principles apply to QS: