str_aggr function not wokring

Hi,
I need to combine results of multiple rows in one row. I get below error.
Could you please help.

Query:

select string_agg((select '******' || P.PhaseName || ' - ' ||
R.Recommendation AS "ABC" from tblAssessmentRecommendation
R,tblAssessmentPhases P
where R.PhaseID = P.PhaseID Order BY P.sortOrder DESC),' ')

Error:

ERROR: more than one row returned by a subquery used as an expression SQL
state: 21000

Regards,
Aditya.