You refer to an "employer-provided 401(k)." However, that TurboTax is calculating a deduction for contributions to a 401(k) seems to imply that you are self-employed (either a sole proprietor or a partner in a partnership) and are reporting contributions to a self-employed 401(k) plan. There is no deduction on your individual tax return for 401(k) contributions made to a 401(k) plan of some other employer (including an S corp of which you might be a shareholder).
Assuming that you are contributing to a self-employed 401(k) plan, in addition to the $19,000 regular employee contribution limit, $6,000 catch-up limit on employee contributions and the employer contribution limit, there is the limit imposed by net earnings from self-employment. If TurboTax is limiting your self-employed retirement deduction to $12,155, this implies that your net profit from self-employment is only $13,079. Your contribution to a self-employed 401(k) is not permitted to be more than your net profit minus the deductible portion of self-employment taxes. If your net profit is only $13,079, it's not clear where the money came from that you used to contribute the $12,930 that TurboTax is showing as an excess contribution since it did not come from your compensation.
Note that if you are partner in a partnership, only your income from the partnership that is net profit from self-employment, shown with code A in box 14 of the Schedule K-1 (Form 1065) is income on which your self-employed retirement contribution can be based. Other types of income from the partnership are not part of your compensation and do not factor into the calculation of your maximum permissible self-employed retirement contribution.
Assuming that you did make a $12,930 excess deferral, that amount is not deductible and you'll pay taxes on that money again when later distributed from the 401(k). Also, the fact that the plan administrator allowed you to make an excess contribution could result in disqualification of the plan.
