You should be able to complete the prior year returns using ProSeries.
This article shows you how to download the software:
https://accountants-community.intuit.com/articles/1606666-download-and-install-proseries-and-proseri...
You want the section for Download From My Account (Current and Prior Year Software).
If 2015/2016 are not listed on your MyAccount you can contact Customer Service at 800-434-6818 to get those added.
The 2016/2017/2018 returns would still be eligible for e-file. 2015 and prior would need to be mailed in.
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