How can I retrieve a copy of my 2016 tax return if I don't remember the username?

[Critter]

You have to sign onto your 2016 online account using the exact same user ID you used to create the account.

Close all TurboTax windows on your web browser (including this one).  Copy and paste the account recovery website link onto a new web browser window and run the tool.

Use this TurboTax account recovery website to get a list of all the user ID's for an email address.  Run the tool against any email addresses you may have used - https://myturbotax.intuit.com/account-recovery

If none of the user ID's received will access your 2016 account, then use the option shown in blue on the account recovery website, "Try something else"