When you are creating suggested messages, certain HTML content is allowed but only when it is created through the Rich Text Editor (RTE). So for example you can underline text in the RTE without issue, but if you copy underlined text from another text editor it will throw this error, because the HTML tags are attempting to copy as well. This tends to happen when admins actually write their message in something like Microsoft Word, and then copy-paste the content into the RTE. The solution is to copy the text you have into a basic text editor such as notepad that will remove any formatting, and then copy and paste it again into Luminate. This will remove the formatting, and then you can format the message within the RTE to look how you want.