If you've used the same TurboTax account that you used when filing for 2017, you should have access to the tax return. Likewise, the 2018 Premier purchase (as long as you've used the same account) should unlock any prior-years' returns. Do you possibly have more than one TurboTax account? If you don't see your 2017 return in your tax documents, it's likely because you've used a different TT account. See the TurboTax FAQ below for instructions on how to proceed.
"Why can't I find my return in Tax Home or My Docs?"
https://ttlc.intuit.com/replies/3290682
If you have purchased TurboTax 2018 in a different account, contact TurboTax Customer Support during business hours for assistance using the info below. After you locate the correct account, stick with the same account.
https://ttlc.intuit.com/replies/3300041