You took a Draw, the same as any taking from business bank for personal use. This value is reported on the 1040 and not the 1065 or Sched C. There is nothing to do about "2018 in 2019" for this taking. You reported it on 1040 for 2018, and Funded it from a draw from the business.
And you should confirm with your own CPA that you qualify for a "Solo 401(k)" plan and that is the forms you filed at that bank. See this topic: