I'll assume that you have not maxed out the Social Security wage limit when combined with wages reported on a W-2. If your net profit from self employment is $18,718, resulting in net earnings of $17,395, the maximum combined contributions you can make to the solo 401(k) is $17,395. So you have $805 of excess contributions.
The deadline for obtaining a corrective distribution was April 15 (not the due date of your tax return), so it seems that it's too late to make the correction, but you should check with your solo 401(k) provider. If corrective distributions are not made by the deadline, the $200 contributed as a traditional elective deferral is not deductible and will be taxed again when eventually distributed. Similarly, the excess $605 contributed to the designated Roth account will be taxable when eventually distributed. This double taxation of the money is the penalty for making an excess contribution. The first money distributed from the Roth 401(k) will be the taxable excess, but because your solo 401(k) provider has no way of knowing that there was an excess Roth contribution, you'll have to track that yourself and pay the tax on the first $605 distributed from the Roth 401(k) with any amount distributed beyond that taxable as an ordinary Roth 401(k) distribution. You'll likely have to submit a substitute Form 1099-R (Form 4852) when you take that distribution since the one provided by the solo 401(k) provider will not show the correct taxable and nontaxable amounts unless you tell the provider the amount of your excess Roth contribution and they track it.
Note that allowing excess contributions to be made can potentially disqualify the plan, so if the corrective distributions are not made by the deadline, you might want to make the distributions of the excess relatively soon:
https://www.irs.gov/retirement-plans/401k-plan-fix-it-guide-elective-deferrals-exceeded-code-402g-li...
