Use the name that your spouse has on a CRA record. For more clarification please contact CRA directly.

Thanks for contacting TurboTax.

What do I need to enter in Last Name section as its mandatory field in Turbo Tax? (My spouse does not have last name registered with CRA)

You can try entering "Not Applicable" for the last name. You may need to take out the space. you can use 000 000 000 for her SIN number.