Did you try signing in to your spouse's MyAccount before trying?
As per the Canada Revenue Agency (CRA) Preparing a tax return for a family member or a friend
Individuals can also use Auto-fill my return on behalf of a family member. If you prepare a tax return for a family member or a friend, you need to:
Each individual should enter their own CRA user ID and password to use this service. Auto-fill my return can be used for the current tax filing year plus the previous six tax years.
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