Occasionally, the app may show you a red message pending notification, despite there being no message pending. This can occur if, last time you connected to the chat, you had a bad internet connection or there was a connectivity problem and the system did not have time to process the reading or receiving of messages.
In any case, this error does not prevent you from reading and receiving messages if someone sends them to you and you are connected to the chat. Just make sure you're connected to the chat for a few minutes and read all the conversations with a message pending notification. You can be sure that the conversation has been read if the red dot disappears from the list and if your messages appear with at least one tick mark (✓).
If you have repeated this step for all the conversations visible in your inbox, but you still see a red dot on the menu, you may have more conversations that are not in view with messages pending. You can retrieve old conversations by scrolling down the message screen and loading more conversations until you see the one with messages pending. Repeat the process to read all your conversations.