You can efile until Oct 15. After that you will have to print and mail it. Online will shut down soon after Oct 15 to get ready for next year. So at least save your return, both the .tax2020 file and the pdf.
If you filed an extension, the tax filing deadline is this Friday, October 15. If you did not file an extension, you have already missed the deadline. You can still file after the deadline, but you will be filing late. If you are getting a refund, there is no penalty for filing late.
Although you can still file after October 15, you cannot do it with TurboTax Online, as VolvoGirl pointed out. If you save your .tax2020 file on your own computer, as VolvoGirl recommended, you will be able to complete your tax return and file it with the CD/Download TurboTax software.