In case you’re sending the message to a Channel/Group you have to insert the TAG ( es. @telementor_ch ) and not the Chat ID.
If you’re working on a Private Channel, you have to change the visibility of the channel on Public, assign it a TAG and then re-set the Channel as Private.
Into telementor then insert the TAG assigned to your public channel. In this way your channel still be PRIVATE, but plus you have a friendly TAG.
P.S. Bot must be Member and Admin of the Group/Channel.