No such channel

When I try and join a channel, I get this message:

No such channel TheChannelName

And other variations on the theme.

The reason is in the message. Channel names begin with the character #, so by example,

/join TheChannelName

will fail

/join #TheChannelName

is what you wanted to type.

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Yes, there are other characters indicating different channel types on different IRCd systems and different networks, but # is by far the most common character that the new user requires.