You used one of the @RANK or @SORT sorting functions incorrectly.   Proper usage is either:

 ATTR_RANK = @RANK( ATTR1[,...,ATTRN);

or,

ATTR_SORT = @SORT( ATTR1[,...,ATTRN);

where ATTR_RANK (ATTR_SORT) receives the ranking (sorting) of attribute ATTR1, with ties being broken by the optionally supplied attributes ATTR2 through ATTRN.  In addition, @RANK and @SORT are only valid in calc sections.  Please refer to the documentation for more details.