Deductible self-employed retirement contributions reduce the net earnings available to contribute to a Roth IRA, so by contributing all of your net earnings as deductible 401(k) and SEP contributions there is nothing left to contribute to the Roth IRA.
Note that if your SEP IRA uses the Form 5305-SEP plan agreement, you are not permitted to currently maintain any other qualified retirement plan (per the SEP plan agreement). Because the employer contribution to the 401(k) plan and to the SEP plan would be the same amount, it generally makes no sense to make the employer contribution to the SEP plan instead of making it to the 401(k) plan. A prototype SEP plan that spells out the combined contribution limits would be needed to be able to make contributions to both the 401(k) and the SEP plan.
