If 2020 TurboTax, which has not yet been updated from the 2019 elective deferral limits, is limiting your maximum total contribution to less than $19,000, it means that your contribution is limited by net earnings and the fact that 2020 TurboTax has not yet been updated is irrelevant.
It seems that your maximum contribution is being limited by net earnings. It's hard to understand what TurboTax is actually telling you without knowing the amount of your net profit and the deductible portion of self-employment taxes, but it seems that even a $4,000 employer contribution exceeds what is permitted because the results from TurboTax imply that your net earnings (net profit minus the deductible portion of SE taxes) are less than $20,000. To maximize your total contribution, you first maximize the regular elective deferral, then maximize the employer contribution, then maximize any catch-up contribution.
