As of April 2022, Roku is officially supported in the following countries:
In reality, however, it is available much more widely, indeed almost anywhere you can obtain a Smart TV and a Roku streaming device to connect to it. However you might need to connect to Roku through a VPN service.
It is important to note that Roku channel developers can lock their streaming services so they only work in specific geographic regions. The only way around this is to access Roku through a VPN.