In what page/node are you including this content? The variable coreNode.id will be replaced with the ID of the current node. So if you have this on a page/node that is at the Community level, the value of coreNode.id will be your high level community, and since that isn't a Board, you will get an exception.
If you include it in a page that is a board, then I think it should work. Alternatively, if you want to put it on a custom page and display the messages from a specific board (e.g. a board with id 'board-1'), you could use the following call:
<#assign messages_list = restadmin("/boards/id/board-1/messages/linear")>If you don't need to execute the call as a privileged user, you could do the following instead:
<#assign messages_list = rest("/boards/id/board-1/messages/linear")>Hope this helps.
