Can you try:
Assign Activity
dtResult =
(From d in yourDataTableVar.AsEnumerable
Where Not d("Country").toString.ToUpper.Trim.Equals("GERMANY")
Where Not {"6","7","8"}.Contains(d("Grade").toString.Trim)
Where Not {"A Plan","B Plan","C Plan"}.Contains(d("Bonus").toString.Trim)
Select r = d).CopyToDataTable
Handling empty filter result: