did you do 2017 online? if so it should be accessible to do an amended return. Amended returns must be paper-filed.
if you used desktop for 2017, contact support to see if they'll push the 2017 app to you
Contact Customer Service
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https://support.turbotax.intuit.com/contact
Did you use the Desktop program or the Online version? Do you have the 2017 program to install?
If you don't have the actual 2017 return file ending in .tax2017 then you will have to re-enter all your info and recreate your original return. So first install the Desktop program and start a new return, go to FILE - NEW. You can use the Forms Mode to recreate your original return. Then when it is exactly the same as what you originally filed, you go though and pretend to file it by saying you will print and mail it so it thinks you did.
Then when each line is exactly the same as the original return you can amend it.