Since the TurboTax online editions were only for tax year 2019 and those editions shut down after October 20, 2020, you could not have filed any tax return using the TurboTax online edition in November of 2020.
Any refund from a 2015 tax return has been forfeited if filed after April 15, 2019 or with an extension after October 15, 2019.
You could not have used any online program in 2020 to file a 2015 return so did you mail in 2019 forms with the 2015 income on them ??? If you did then expect an IRS letter in the next 6 months and not a refund.
"I did my return online...."
If you used the 2019 online software and entered 2015 income information into it and then mailed it, what you did was file a VERY incorrect 2019 tax return. The IRS might catch the mis-match between the income you reportted and send you a letter. If they actually process it and send you a tax refund--you are not entitled to that refund and must send it back. It is too late to receive a tax refund for any tax year earlier than 2017.
If you just mailed it in November it will take quite awhile before you hear anything. It is taking a very long time for the IRS to process mailed returns.