From the field names I would guess that all of those values are numbers, but you are trying to insert the combo values as text items ?
If my guess is correct remove all the ' ' around your combo concatenated values.
Edit: Form your string as a variable to assist debugging as well;
Code:
Dim sSql as string
sSql = "INSERT INTO Paper_Marking (Lecturer_ID,Unit_ID,No_Of_Papers,Semester_ID) VALUES ('" & Me.Combo2 & "','" & Me.Combo5 & "','" & Me.Text11 & "','" & Me.Text8 & "')"
Debug.Print sSql
Currentdb.Execute sSql