I think we have Cicero's definition through Torquatus (We are inquiring, then, what is the final and ultimate Good, which as all philosophers are agreed must be of such a nature as to be the End to which all other things are means, while it is not itself a means to anything else)
And that is the problem. We don't have Epicurus endorsing that specific formulation.
This formulation presumes that we have the ability to discover something that completely and accurately fulfills this definition for all times, all places, all people. I suspect rather strongly that that is not possible. THE end? Why should we presume that there is only one? Or that it is the same for all?