Error You must be creator, or administrator
In case the user connects the channel to the Telepost through the bot @TelepostBot, and after that the user leaves the channel or stops being admin / owner in it then when trying to connect such a channel, the error You must be creator, or administrator will appear in the Web UI:

Solution
The user who connected the channel through the bot @TelepostBot needs to become an admin / owner in the channel again.
It is also recommended to use your own custom bot, how to connect it - read in this manual.