The $18,000 annual limit is per individual. TurboTax is incapable of enforcing the $18,000 per-individual limit for elective deferrals and Roth contributions to an Individual 401(k) when you have also made such contributions to employer plans.
As a workaround in TurboTax, do not mark the Maximize box for an Individual 401(k). Instead, enter only the remaining $7,000 of elective deferral or Roth 401(k) contribution on the Individual or Roth 401(k) page, then on the following pages mark the Maximize box for a Keogh Profit Sharing Plan. This will cause TurboTax to calculate and include the maximum permissible deductible employer contribution.
