I can't download my 2018 ".Tax" file--it just takes me back to the login screen instead of downloading it.

[Critter]

You need to register a 2019 online account to access prior years
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/01/270...

[Charles2018]

I already did that and still no luck.

[Critter]

Contact TurboTax support and speak directly with a TurboTax support agent concerning this situation.

See this TurboTax support FAQ for a contact link and hours of operation -https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number