SQL Server - Error La subconsulta ha devuelto más de un valor

select ISNULL(ltrim(rtrim('<set value=''')) + ltrim(rtrim(CANT))  +

ltrim(rtrim('''/>')),'<set value=''''/>')

from FTE_COB_CO

 where FTE ='CC' and HRS = '00' AND COB = 'A'

SELECT CASE WHEN not exists(

SELECT CANT from FTE_COB_CO

WHERE FTE ='CC' and HRS = '00' AND COB = 'A' )

THEN '<set value=''''/>'

ELSE

(select ISNULL(ltrim(rtrim('<set value=''')) + ltrim(rtrim(CANT)) +

ltrim(rtrim('''/>')),'<set value=''''/>')

FROM FTE_COB_CO

WHERE

 FTE ='CC' and HRS = '00' AND COB = 'A' ) END

Set @pEstado =case when ltrim(rtrim( @pLinTemp))  = ltrim(rtrim( @CHR_COD_LINEA_ADRYAN))

        then  'Si'

    else

        Case when ltrim(rtrim(@CHR_COD_LINEA_TMP)) = ltrim(rtrim(@pLinTemp))

                then 'Si'

                else

                'L'+ltrim(rtrim(@pLinTemp))

        end

    end