[NOTE: I enter a self-employed income of $63,000 and TurboTax calculated and accepted a contribution of $18,500.
If you're still having difficulty, please respond to this thread with additional details.
If you are an employee:
Contributions to your 401(k) are entered on your W-2. The amount is shown in Box 12 with code D. It is the reason is box 1 is lower than box 3.