I am wondering whether I have to write the paper regarding Canada or some other countries as well.
As a native speaker, this sounds... off. First off, the word "regarding" doesn't seem to fit here; the word "about" would sound more natural. "I'm asking a question about..." flows better than "I'm asking a question regarding..."
Then, going into "some other countries as well". The "as well" makes it sound like you're asking if it's just about Canada or Canada and other countries. You're probably looking for "instead" here. (If you're not, then leaving "as well" in there will work.)
Since you're wondering whether it has to be about Canada in particular, adding a "specifically" after "Canada" makes that clearer.
Then, this is optional, but adding in an "or if" makes it flow better to a native ear (at least in my opinion), although it's not strictly necessary. Once you're doing that, adding in what you're doing again (writing) makes it sound more correct.
The word "some" is unnecessary here; it's not serving any purpose.
So, if we take all of that into account, we get something like this...
I'm wondering whether I have to write the paper about Canada specifically, or if I can write about other countries instead.
...which sounds much more natural to my ears.