How can I amend my 2017 tax return without the .tax2017 file? I have tried to download it multiple times and keep receiving the 'Failed - Network error' message.

Try downloading it using a different browser.

Or You will have to first recreate your original return exactly as filed.  Then when it matches each line you say you want to print and mail it and print it out so Turbo Tax thinks you filed it.

Then you can do the amended return.

you also need the 2017 software.  you can not use 2018