SQL Server Error: Subquery returned more than 1 value

why im seeing this error..???

Msg 512, Level 16, State 1, Line 3Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

            DECLARE @AdmissionNo VARCHAR(10)           
            ;WITH cteStudentMarksDetails
                AS (
                select ts.AdmissionNo
                    ,TestDate
                    ,SubjectName
                    ,MaxMarks
                    ,Marks
                    ,Round((Marks) * 100/ (MaxMarks),1) as SubjectPercentage
                    ,DENSE_RANK() over(partition BY (SELECT MAX(Marks) FROM tblTestSystem TSS Where Tss.SubjectID = ts.SubjectID AND Tss.ClassID = ts.ClassID)  order by ((Round((SELECT Marks FROM tblTestSystem TSS Where Tss.AdmissionNo = ts.AdmissionNo AND Tss.SubjectID = ts.SubjectID AND Tss.ClassID = ts.ClassID) * 100 / (SELECT MAX(Marks) FROM tblTestSystem TSS Where Tss.SubjectID = ts.SubjectID AND Tss.ClassID = ts.ClassID),1) )) DESC) AS SubjectPosition
                    ,CASE WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 80 THEN 'A+'
                          WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 70 THEN 'A'
                          WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 60 THEN 'B'
                          WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 50 THEN 'C'
                          WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 40 THEN 'D'
                          WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 33 THEN 'E'
                          ELSE 'FAIL' END AS Grade,
                          CASE WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 80 THEN 'OutStanding'
                          WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 70 THEN 'Excellent'
                          WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 60 THEN 'Very Good'
                          WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 50 THEN 'Good'
                          WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 40 THEN 'Satisfactory'
                          WHEN  Round((Marks) * 100/ (MaxMarks),1) >= 33 THEN 'Work Hard'
                          ELSE 'FAIL' END AS Remarks   
                    ,(Round((SELECT SUM(Marks) FROM tblTestSystem TSS WHERE Tss.AdmissionNo = ts.AdmissionNo) * 100/ (SELECT SUM(MaxMarks) FROM tblTestSystem TSS WHERE Tss.AdmissionNo = ts.AdmissionNo),1) ) TotalPercentage
                    ,DENSE_RANK() OVER(ORDER BY ((ROUND((SELECT SUM(Marks) FROM tblTestSystem TSS WHERE Tss.AdmissionNo = ts.AdmissionNo) * 100 / (SELECT SUM(MaxMarks) FROM tblTestSystem TSS WHERE Tss.AdmissionNo = ts.AdmissionNo),1) )) DESC) AS ClassPosition
                FROM tblTestSystem AS ts
                INNER JOIN tblStdReg AS sr ON ts.AdmissionNo=sr.AdmissionNo
                INNER JOIN tblDefClass AS dc ON ts.ClassID=dc.ClassID
                INNER JOIN tblDefSection AS ds ON ts.SectionID=ds.SectionID
                inner join tblAssignSubjects as si on si.ASID=ts.SubjectID 
				inner join tblDefSubject as des on si.SubjectID=des.SubjectID
                
                )
 
                SELECT * FROM cteStudentMarksDetails