"Error in Template:" message when creating a new Custom HTML Block

When I just opened the Custom HTML Block interface then clicked create new. As soon as the newly created interface appeared, I saw the web console with the message:

root element is already there and instruction are available at custom HTML block type how to deal with it

I want to know what that token is and why it shows up. Because when I just clicked on creating a new HTML block and hadn’t written any more code, I got this message.