"I have a net profit of $19,771 on self-employed income. I entered $18,374 solo 401k contribution and $3,652 profit sharing in Turbotax."
What you described results in an excess contribution. Your total additions to the solo 401(k) (employee plus employer contributions) are not permitted to exceed net earnings from self-employment. Net earnings are net profit minus the deductible portion of self-employment taxes, so with $19,771 of net profit, net earnings are $18,374. Your employee elective deferral of $18,374 uses up all of your net earnings, leaving nothing left to contribute as an employer contribution and making the $3,652 profit-sharing employer contribution an excess contribution.
