It seems a normal question and I have searched and tried many suggestions here but the error persists. I want to copy a \"case1\" sheet from current workbook to an existing work
Always set references to sheets and workbooks, then there is no chance of confusing which is which. This code does the same thing and should not give you any errors.
Sub CopySheet()
Dim wb1 As Workbook, ws1 As Worksheet 'Source
Dim wb2 As Workbook, ws2 As Worksheet 'Target
Set wb1 = ThisWorkbook
Set ws1 = wb1.Sheets("case1")
Set wb2 = Workbooks.Open("c:\workbook2.xlsx")
Set ws2 = wb2.Sheets("case2")
ws1.Copy Before:=ws2
wb2.Close True
End Sub