Last year, I entered YES in error - and I could not remove it and then I had to follow up with CRA to have it corrected after filing. I informed them that I was unable to turn it off in Turbo Tax;which I had used last year (2017) as well
I read somewhere that I could try to go to Schedule 1 and make sure Box 316 is set to zero -- but in TurboTax Online software, how can I do that?
Unfortunately you don't have the use of forms in Online version of TurboTax.
You might want to simply start your return over again- but this time enter everything yourself. Do not forward your information from last year. Especially if you tried to remove it and it won't remove. Usually when someone has DTC it stays for years.