I completed my 2018 return using desktop software. Is it possible to upload this file to TT on-line as a backup or to use data in next years return by the online version?

Yes, you can. You can upload your completed 2018 tax file created by the desktop program into TurboTax Online next year.

Please read this answer from TurboTaxElisabethB2:

https://ttlc.intuit.com/replies/6246284

Is it possible to upload the 2018 return now, or must I wait until starting the 2019 return next year?

No, you cannot. You can only upload a completed return, not a return in progress. There is no point in uploading a completed 2018 tax file into the 2018 TurboTax online.

Thank you