There is no such thing as a "SEP 401(k)." SEP plans are IRA-based plans, not 401(k)-based plans. Perhaps you are referring to a self-employed, solo or individual 401(k) (all different names for the same thing).
The answer below applies to your situation the same as it did to the person who posted the original question. To get TurboTax to calculate only the maximum employer contribution use the Maximize function for either a SEP contribution or a Profit Sharing Keogh, not the Maximize function for a 401(k). You are not permitted any elective deferral to any type of self-employed retirement plan because you've already made the maximum permissible total elective deferral for 2018.
