RUN Time error,Rename PDF files using Excel Macro

Your fpath is already imbedded into both of the strings in columns A:B, so you don't need to parse it out.

Sub Rename_All()
Dim LastRow As Long, Rw As Long

    LastRow = Cells(Rows.Count, "A").End(xlUp).row
    For Rw = 1 To LastRow
        Name Cells(Rw, "A").Value As Cells(Rw, "B").Value
    Next

End Sub