How can I access my 2018 tax return from Turbotax Online?

This should help:

https://turbotax.intuit.com/tax-tips/irs-tax-return/how-to-obtain-a-copy-of-your-tax-return/L13d9uLMa

Please follow the steps in the below TurboTax FAQ to access your 2018 tax return in TurboTax Online:
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/00/27010