@AllanHZ
1) review your transcript on IRS.gov. Is it possible it was paper filed "way back when" and you don't remember that? is the balance for 2018 zero'd out? Any record on the transcript that the tax return was processed?
2) if there is no record of a return on IRS.gov, you can't amend. The original tax return would have to be submitted.
3) do you have a copy of the 2018 return? if necessary, you can print, sign and mail it in.
4) if it turns out that in deed the IRS has no record of your 2018 tax return, here is the further problem. if you were due a refund, the window to receive that refund expired in April, 2022. I'd still send on the 2018 tax return, but don't expect to receive the refund. if you owe money, send it along with the tax return.
