This works for me.
Code:
Sub Update_Log_Status()
Dim wrkbk1 As Workbook
Dim sht1 As Worksheet
Dim row1 As Long
Dim mywb As Workbook
Dim sht2 As Worksheet
'Path to closed file
Const filePath1 As String = "J:\Manual POs\Manual PO Log\Manual PO Log2.xlsx"
'Const filePath1 As String = "D:\Forum Stuff\2017\2017_03_01\CLoos\Manual PO Log2.xlsx" '<~~ test path
'Stop updating the screen
Application.ScreenUpdating = False
Set mywb = ActiveWorkbook
'Open the workbook
Set wrkbk1 = Workbooks.Open(filePath1)
'Grab the first sheet in wrkbk
Set sht1 = wrkbk1.Sheets(1)
mywb.Activate
Set sht2 = ActiveWorkbook.Sheets("PO")
'Find value in B21 of active worksheet in colum D of closed workbook
On Error Resume Next '<~~ incase not found
row1 = sht1.Range("D:D").Find(sht2.Range("B21").Value, , xlValues, xlWhole).Row
On Error GoTo 0 '<~~ turn error notification back on
If row1 > 3 Then
sht1.Cells(row1, 10).Value = "A"
End If
'Close workbook & save
wrkbk1.Close True
'Set the screen to update
Application.ScreenUpdating = True
End Sub