Did you use TurboTax Online or did you use TurboTax CD/Download?
There are a few reasons you might not be able to find the TurboTax Online return you’re looking for. https://turbotax.community.intuit.ca/replies/2648026
Here is some information that may help you find your files if you used TurboTax CD/Download (2018 is .tt18): https://turbotax.community.intuit.ca/replies/3285180
Here is how to submit your tax return to CRA using the NETFILE service in TurboTax CD/Download: https://turbotax.community.intuit.ca/replies/2647647
Here is how to submit your tax return to CRA using the NETFILE service in TurboTax Online: https://turbotax.community.intuit.ca/replies/3210262