I have the same problems from BC. I contacted Cineplex, and they say that the channel is definitely supported in my province. They indicated that the issue was likely with how my Roku was initial setup, but didn't know how I could fix it.
I ended up having to do a factory reset on my Roku, and creating a new Roku online account. Once I re-registered my device I was able to add the Cineplex channel. What a PITA.
I just installed a new Roku Ultra upgrading from the USB stick. Very disappointed that there is no Cineplex Canada channel! Very Upset!
Followed the suggestion to create an new online account plus reboot device and start from scratch! Jeeesh