Updated at 25 Jan 2022
Telegram channels are broadcasting platforms for the users, usually big audiences. The same is for groups, where people can share their messages too, not only channel or group administrators. And if you try to join and get "this channel cannot be displayed" error messages, then it seems that this channel was restricted by the Telegram team for users in your country or in many countries.
The reasons behind the error are unclear. Telegram doesn't cover this question on their FAQ page.
Probably, it's due to sensitive content on the channel, which may be not safe for work. Telegram stays strict behind its rules and it moderates channels and groups for such content.
If a channel seems to be not safe to read, Telegram moderators restrict it from the public. It doesn't mean the channel is banned. It's a visibility restriction. Hence, people in some countries, or, people in general won't be able to see it.
"this channel cannot be displayed" is a general error and there are some others that mean the same in different contexts. The other possible errors are:
There are two ways of doing so. The first option is a switch in the Telegram Desktop app settings. Yes, mobile versions of it don't have one. To find the switch, follow the following steps:
Sometimes Telegram tries to restrict a channel/group's visibility to specific countries due to reports from some people or companies. Try to change your Internet traffic origin by changing countries in some VPN app. Then try to access the channel/group again.
It's not all! We can't restore visibility for a specific channel or a group, as well as for other people.
There are websites that collect Telegram channels, their statistics, and posts. Often, you can find the channels that give you the " this channel cannot be displayed" error there. It's like catalogs of websites where you can pick a website by category. The same approach here, but you can go directly to the channel's page.
For example, Telegram Tips - an official Telegram channel, which is also can be found on this catalog.
You can find other channels that "blocked" you from seeing them on such websites. Search them in Google.