This feature is currently being tested in version 2.23.12.20 of WhatsApp Beta, which is available via the Play Store to those enrolled in the beta program. It turns out that when the Channels feature launched, users that had access to it shared invite links which ended up in the hands of users that did not have access to the feature. At first, this resulted in a generic message stating that the feature is not yet available.
Source – WaBetaInfo
It is unclear when channels will be available to users outside of Singapore and Colombia. However, the fact that WhatsApp is working on a feature to allow users to be notified when channels are available suggests that they are planning to roll out the feature to a wider audience in the near future.