If you have the tax return information available from the last time you filed, go ahead and try the online system since it says to use your most recent return. They use this information to verify your identity. If you don't have that information or they do not accept it, you will have to send in Form W-12 with copies of you Social Security card, and your photo ID.
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You did not say how long it has been since you have filed, but if you have the information and have not changed your name, I would try it. The IRS did not elaborate on "most recent", so I hope this works for you. Otherwise, it could take 6 weeks to get your PTIN.