Thank you for clarifying, Nicole.
The problem here is your location. Even though you may be using her appletv account, the content server (pbs, in this case) is able to detect your location, via the IP address assigned by your internet provider. This is a purposeful content restriction based on country, and is very common, not just to pbs but to other apps too. If you went into netflix at your location and again at hers, you'd find different offerings for example.
There is no quick or simple setting you can change for that. The only way around it would be to pay for a VPN or proxy service which essentially reroutes your appletv connection signals and tricks the server into thinking you are in another location. When you do that and it then thinks you are in the US, you may then miss out on some CA offerings however.
This process can be expensive and complicated (you really need to be very computer and IT savvy to configure this type of setup). I can give you some information on this if you'd like, but in most situations all you can really do is accept the location limitations or find another way to access the content elsewhere.
Best regards, ***** ***** for the news,