Did you use the same User ID to sign in with as last year?
You can transfer from 2015 on your computer. You need to have the file ending in .tax2015 on your computer. Here's how to transfer it to online version.
Go to My Account > Tools > Transfer last year's TurboTax return from your computer > Continue
If you used the software CD / Download last year and you are trying to use Online this year your your CD / Download files on your desktop will not transfer to the Online product because Turbo Tax CD / Download is only transferable to the desktop version (CD / Download). Online can be saved as a .taxfile2016 on your desktop, however, to be transferred to CD / Download.
Here's the FAQ on transferring. You can transfer from Desktop TO Online
https://ttlc.intuit.com/replies/3301662