how to convert ARRAY>> to string in big query

Question

MERGE table1 M USING (

select  _id,ARRAY(SELECT AS STRUCT value from unnest(preferences) as p where p.name="sendOnly")  as prefer from table2    

) S ON M._ID = S._ID WHEN MATCHED THEN 
UPDATE 
SET 
M._ID = S._ID, 
M.preferences_testing=s.prefer  // here getting error
WHEN NOT MATCHED THEN INSERT (
  _ID,    
  preferences_testing
) 
VALUES 
(
_ID,   
prefer
)

getting error while converting the array struct to string , I tried to convert ARRAY<STRUCT> to String but nothing worked