Can I complete a tax return for a dependent under my log in a password? I have used my log in and password to complete my return.

You will need to create/use a separate TurboTax account to file a return for your dependent.  Here is another help article that can walk you through that process.

https://ttlc.intuit.com/questions/1894512-how-do-i-start-another-return-in-turbotax-online?jump_to=a...