I entered a wrong SIN for my partner in our joint declaration. I was able to submit mine, but can't submit her's or modify the SIN. Anyone has insight for me?

 Once a tax return is submitted, it cannot be changed. However, if you are using TurboTax Download, you can try going to EasyStep, then on the left-hand side menu go to INFO and continue until you get to the page where your SIN number is entered to see if you are allowed to edit it from there. If not, you will need to print and mail the return or start a new return. Once you print and mail the return, you will be able to fix your SIN number before mailing it out.

It's important to note that if there is an error in your spouse's SIN number, you will need to contact the CRA at 1 (800) 959-8281 to let them know about it since it will show up on your return.

Will I need to pay a second time then? I paid for 2 declarations.

Not if you click on Print to Mail- that is the same as filing a return.