Message box macro not working

I'm not sure why you have a for loop running on one cell?

replace you code with this..

If Intersect(Target, Range("D8")) Is Nothing Then Exit Sub
If (Range("D8").Value < Range("F8").Value - 0.02) Or (Range("D8").Value > Range("F8").Value + 0.02) Then
       MsgBox "SCALE CHECK 1 HAS FAILED CALIBRATION AND MUST BE RECALIBRATED BEFORE IT CAN BE USED TO TAKE WEIGHTS"
             Exit Sub
End If