I can't watch Netflix on my Roku. It is because my old Roku device need an update but unfortunately is no customer service number to call for help with this matter
You need a new roku, netflix isn't upgrade older roku's anymore. If you have roku account, check your upgrade page for any offers:
What appears on that page is different for each roku customer. It depends on the model of roku's you have on your account.
Since you didn't provide your Roku model number, we can't truly say why Netflix isn't working for you. But your description makes it sound like one of the first couple of generations of Roku, and yes Netflix support is pretty thin for devices built 2013 or before.