SEP contributions cannot be Roth contributions.
It's extremely common for people to think that a Roth account in an employer plan is a Roth IRA, but it can't be. A designated Roth account in an employer-provided retirement plan is a different kind of Roth account.
Roth contributions to an employer-provided retirement plan are reported with code AA, BB or EE in box 12 of the W-2 and are to be entered only in box 12 of TurboTax's W-2 form.
Employers can provide for deposit of part of your pay into a Roth IRA, but this is simply a convenience for you to make regular personal contributions to a Roth IRA and is not an employer-provided retirement plan. It's unlikely that the amount of personal contributions would appear anywhere on your W-2, but if it did, it would only be for your own information, perhaps in box 14.
