I've shown my Employer, who uses ADP, this IRS site and sent them the general instructions for 2018 W-2 preparation, both of which are clear the employee needs to be an 'Active Participant' (p. 22), but they won't fix it. Even the chart on p. 30, second row, describes my situation.
https://www.irs.gov/retirement-plans/common-errors-on-form-w2-codes-for-retirement-plans
The marking of box 13 Retirement plan on your W-2 is quite possibly correct. It's possible that the employer made additions to your account in 2018 (say, from forfeitures from employees who separated from service before being fully vested in company matching contributions) or it's possible that additions were made to your account in 2017 but for the plan year ending in 2018 due to the plan using a fiscal year instead of a calendar year. The IRS goes by the marking of box 13, so you must enter the W-2 as received.
There were no additions, from anybody, in 2018. I started in mid-November, 2018.
The employer offers only a defined contribution plan (401k), with a match. The account was $0 on 12/31/2018.
You would have to ask the employer. If checked in error then the employer needs to issue a corrected W-2 so the IRS gets the correction.
Thanks. As I said in the Details, I've shown them the IRS Instructions for W-2 and they won't acknowledge the error. Which is why I asked: "How should I proceed?"
As dmertz said, the IRS goes by what the employer puts in box 13. If you uncheck the box on your own then you can expect an IRS audit letter about this time a year from now. Your employer should be able to explain the reason the box is checked.
If you weren't employed with the company in 2017, the company doesn't provide a defined benefit plan, you made no elective deferrals or Roth contributions from your 2018 pay and the employer made no profit-sharing contribution for 2018, this does seem to suggest that box 13 retirement plan should not have been marked on your W-2.