How can I get my 2016 taxes if I was under another account at that time and I can't remember my username and password for that account

Many people have multiple TT accounts and forget how to access them.  Log out of the account you are in now.

Account recovery

https://myturbotax.intuit.com/account-recovery/

https://ttlc.intuit.com/questions/1901535-forgot-your-turbotax-online-user-id-or-password

Or did you use the desktop version of TurboTax?  If so, the files are on your own hard drive or any backup device you used like a flash drive.

https://ttlc.intuit.com/questions/1901486-how-many-turbotax-accounts-do-i-have

https://ttlc.intuit.com/questions/1901535

https://ttlc.intuit.com/questions/1901659-find-your-tax-data-file-in-mac

https://ttlc.intuit.com/questions/1900721-find-your-tax-data-file-tax-file-in-windows

Access to prior year returns

https://ttlc.intuit.com/questions/3355524-when-does-access-to-my-prior-year-turbotax-online-returns-expire

https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/00/27010/message-id/746

You can get a free transcript from the IRS:   

https://www.irs.gov/individuals/get-transcript

How do I get a copy of a return I prepared using desktop TurboTax?

https://ttlc.intuit.com/questions/1900639

Note:  Every year, before mid-October, print a copy of your current year tax return and save it as a tax file and pdf, too.  That way you will have it when you need it for school, a loan, next year’s tax return, etc. without going through a lot of aggravation (and expense)  to get it.

https://ttlc.intuit.com/questions/1899645-how-do-i-save-my-2018-turbotax-online-return-as-a-tax-data-file

https://ttlc.intuit.com/questions/1912544-how-do-i-save-my-2018-turbotax-online-return-as-a-pdf

My 2016 return is under [Pii Removed] when I filed that year. I need proof to send IRS cuz they still have refund for that year. It is for $1109.00 . How can I get that year 2016 return? I filed 2017, 2018, n 2019 in the same account.