Inner Joins, Coalesce and Pivot

Question

Can someone help me with this error?

  Below is the code:

declare @cols varchar(1000)
declare @sqls varchar(2000)

select @cols = COALESCE(@cols + ', ','') + QUOTENAME(t.TermCode)
from tblFinalGradeDetails fg INNER JOIN tblTermMaster t ON fg.TermId = t.TermId

Group by t.TermCode
SELECT @sqls = 'select *
      from (
        select fg.StudentID, fg.StudentName, t.TermCode, fg.FinalGrade
        from tblFinalGradeDetails fg INNER JOIN tblTermMaster t ON fg.TermId = t.TermId
        WHERE fg.SchoolCode = ''ISL''
        and fg.SchoolYear = ''SY201718''
        ) src
        PIVOT (
          sum(fg.FinalGrade)
          FOR t.TermCode
          IN (' + @cols + ')) pvt '
EXEC(@sqls)

I don't have any problems or issues without the joins but with the joins, I got that error. Thanks so much for any extended help.