You might try shortening it a bit:
Dim wbkImport As Excel.Workbook
Set wbkImport = Application.Workbooks.Open(Filename:=Range("D49") & Range("B49"))
wbkImport.Worksheets("Sheet1").Range("A1:II1200").Copy _
Destination:=Workbooks("Existing Workbook.xlsm").Worksheets("Stress").Range("A1:II1200")
wbkImport.Close SaveChanges:=False
Set wbkImport = Nothing
You also might want to be specific with the two Range statements, ensuring you are referring to the right cell, something like:
Filename:=worksheets("Master").Range("D49") & worksheets("Master").Range("B49")
Assuming the cells are on your Master worksheet.
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