Invalid or unqualified reference error when inserting a formula into a cell via VBA

Hello everyone. I am attampting to insert the following formula into a cell:

sum('[Client_Operations.xlsm]Client_Operations_Details'!$L$2:$L$500)

I am using the code below to do this, and I am receiving an "invalid or unqualified reference" error.

Sub Calculate_157_Disclosures()
    Dim sPath1      As String
    Const sFileInp1 As String = "157_Disclosure.xlsm"
    Const sFileInp2 As String = "Client_Operation.xlsm"
    Dim wb1         As Workbook
    Dim wb2         As Workbook
    
    sPath1 = fPath & fDate & "_157\157_Reports\IBRD_Disclosure\"
      
    Set wb1 = Workbooks.Open(sPath1 & sFileInp1)
    Set wb2 = Workbooks.Open(sPath1 & sFileInp2)
    
    .Range("I14").Formula = "=sum('[Client_Operations.xlsm]Client_Operations_Details'!$L$2:$L$500)"
    
    wb1.Close SaveChanges:=True
    wb2.Close SaveChanges:=True
   
End Sub

Any suggestion on what is going wrong?