The column prefix 'a' does not match with a table name

Question

 DECLARE @cols AS NVARCHAR(MAX),
 @query  AS NVARCHAR(MAX)
 select @cols = STUFF((SELECT ',' + QUOTENAME(role_id)
  from form_appr_workflow where ref_no='LYC002-001-CHAR-2018-96'
   group by user_id, role_id
   order by role_id
   FOR XML PATH(''), TYPE
   ).value('.', 'NVARCHAR(MAX)')
        ,1,1,'')
set @query = N'SELECT ' + @cols + N' from
(
                select user_id, role_id
                from form_appr_workflow
            ) x
 pivot
            (
                max(user_id)
                for role_id in (' + @cols + N')
            ) p '

exec sp_executesql @query;

Result

column-2068     2069     2055

value-   usr1      usr2      usr3

 I need to join one more table to the existing table on roleid ,because in the result instead of numbers(like 2068 etc) i need related rolenames

 

Iam trying to do like this

 it is saying error like 

SQL Error (107): The column prefix 'a' does not match with a table name or alias name used in the query. */ 

DECLARE @cols AS NVARCHAR(MAX),
 @query  AS NVARCHAR(MAX)
 select @cols = STUFF((SELECT ',' + QUOTENAME(a.role_id)
  from form_appr_workflow a join win_role_master b on a.role_id=b.role_id where a.ref_no='LYC002-001-CHAR-2018-96'
   group by a.user_id, a.role_id
   order by a.role_id
   FOR XML PATH(''), TYPE
   ).value('.', 'NVARCHAR(MAX)')
        ,1,1,'')
set @query = N'SELECT ' + @cols + N' from
(
                select a.user_id,a.role_id
                from form_appr_workflow a
) x
 pivot
            (
                max(a.user_id)
                for a.role_id in (' + @cols + N')
            ) p '

exec sp_executesql @query;

  Expected result

column-coo        chief     head

value-   usr1        usr2    usr3