An after-tax contribution to a 401(k) is not an elective deferral. Elective deferrals are the contributions that are subject to the $20,500 limit (for 2022). TurboTax is unable to handle the calculation of your maximum permissible contribution if you have made elective deferrals or Roth contributions to another employer's plan.
You'll have to do the calculation of the permissible employer contribution and after-tax contribution yourself. In the absence of an after-tax contribution, you could use the Maximize function for a SEP contribution to calculation the maximum employer contribution, but when you make an after-tax contribution, which is reportable nowhere on your tax return, you yourself must also make sure that the sum of all of the contributions you make to the solo 401(k), both as employer and as employee, does not exceed the net earnings from self-employment.
