An item in the LINKAGE SECTION has been referenced in the source code, but has no addressability.
A linkage item has to be given addressability before it is referenced, either by a SET statement or by being passed as a parameter. If the code flow at run time is such that addressability has not been assigned, then you receive this error at run time.
Additionally, if the program does not contains any possible way for addressability to be given to that item (such as the item is not a parameter, or not referenced in a SET statement at all) then, the Compiler already has the information at compile time that whatever the code flow is, the item will not be assigned.