import data from access table to excel pivot table - Enable Auto Refresh

You can use the Kill command to delete an existing file, if that helps.

Something like:

strFile = "C:\folder1\workbook_name.xls"
If Dir(strFile) <> "" then Kill strFile

That should be fine, but you may as well use the file variable in the OutputTo method too:

Function Macro11()

strFile = "C:\Users\okl\Desktop\book.xls"
If Dir(strFile) <> "" Then Kill strFile

On Error GoTo Macro11_Err

    DoCmd.SetWarnings False
    DoCmd.OutputTo acTable, "sdf", "MicrosoftExcelBiff8(*.xls)", strFile, False, "", 0


Macro11_Exit:
    Exit Function

Macro11_Err:
    MsgBox Error$
    Resume Macro11_Exit

End Function

no luck...still prompt to overwrite exit file

I have to assume that it's not that code that is being run when the message appears then - the DoCmd.SetWarnings code should be suppressing the prompt already.

It works for me (always has, and I just tested again using your code, but with different file path). Put a breakpoint in the code (select a line and press f9) and then run it as normal just to make sure this is actually the code that is being run!

in the Access->Tools->Macro->Convert Macros to Visual Basic

Then i copy and paste the code in right?

Yes - how are you actually running the code?
You may also want to try this version:

Function Macro11()

strFile = "C:\Users\okl\Desktop\book.xls"
If Dir(strFile) <> "" Then Kill strFile

On Error GoTo Macro11_Err

    DoCmd.SetWarnings False
    DoCmd.OutputTo acTable, "sdf", acFormatXLS, strFile, False, "", 0


Macro11_Exit:
    Exit Function

Macro11_Err:
    MsgBox Error$
    Resume Macro11_Exit

End Function

No. You currently have two versions of your macro - the original Macro1 in your Macros list (which I suspect is what you are running) and the version that has been converted to VBA. How do you want/intend to run this in future?
I am attaching a version of your file in which I have altered your Macro1 macro so that it runs the VBA version of the original macro instead. Let me know if that works for you?