If your HMI screen is trying to subscribe to a tag that does not exist on the server, this red box shows up. This issue typically occurs when an HMI screen is reused from other projects or tags are mistyped.
Move your mouse over the red box and note the tags listed in the window. Try to find these tags in your HMI screen and correct the tag names or add the missing tags to the server.
Please also note that while the Real-time viewer is running, click on Ctrl-F3 to see a window with a list of all tags that the current screen is using. Any tag that cannot be subscribed is highlighted in red.