When posting a new job and you are setting your location preferences you have two options (1) USA only and you can add your preferred time zone and (2) worldwide add your preferred countries. At this time, is there no way to only capture Canada and USA freelancers only?
You can select the US only and that will be a requirement, which means freelancers outside of the US won't be able to see the job.
You can also set preferences for the job and those can be for any country. That will show on the job and inform freelancers about your preferences but freelancers will still be able to apply. You can check the "Can I post a job for a particular country?" section of this help article for more information.