How do you resolve “Content is invalid, merge tag or block tag contains error”?

If you copy and paste any links in like this {{http://www.URL.com}}  then you will get this error message:

The correct way to set the merge tag is this: {{tag_name}}

You can try this method if you want to include links in your campaign without the merge tags:

Please Note: Using the wrong merge tag will trigger this error: “Content is invalid, merge tag or block tag contains error.