I don't know exactly what the TurboTax screen said, but the deadline to get an additional refund for 2015 was April 15, 2019.
If you want to, post your screen shot and we'll take a look at it. Be sure to remove any personal information. This is a public web site. Is the screen shot from the 2015 TurboTax software?
It's not worth appealing because you won't win. The deadline is firm.