A 2018 tax return can no longer be electronically filed, it must be printed and mailed. The TurboTax online editions for tax year 2018 were permanently closed after October 20, 2019.
To complete a 2018 return started with the online editions you would have to purchase, download and install on a personal computer one of the 2018 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
Since you live outside the US, then after the tax return is printed, signed and dated you probably would want to mail the return using a private delivery service such as DHL, UPS or FedEx, instead of using the local mail system.
Use this address if using a private delivery service -
Austin - Internal Revenue Submission Processing Center
3651 S IH35,
Austin TX 78741
