You can efile until Oct 15. After that you will have to print and mail your return. You have 3 years to file for a refund. If you need to finish after Oct 15 you will have to switch to the Desktop program. Online will be shutting down soon after Oct 15 to get ready for 2020.
Any tax due you owe was still due July 15 for 2019 returns.
if you use US mail it must be postmarked 10/15 or earlier. dropping it into a mailbox just before midnight on 10/15 would probably result in a late return because it won't get to a PO for postmarking until the following day. however, if you are due a refund it won't matter unless the IRS were to change your return so you have a balance due.