If you stay with the function, the reason it is failing is that you are only passing in two variables but the function requires three variables.
Code:
Sub DocOps_SPD_Rewrites()
Call Simple_FindReplace(ActiveDocument, "BEFORE text", "AFTER text")
Call Simple_FindReplace(ActiveDocument, "BEFORE test", "AFTER test")
Call Simple_FindReplace(ActiveDocument, "BEFORE tent", "AFTER tent")
End Sub