What is the "Ooops" message and how can I reply?

Why did I receive this message?

• To let you know that you have replied incorrectly to a text message you have received. 
• To let you know that the text message you have just sent has not been received.
• Your reply should follow the correct format as stated on the original text message. The format is <SHORTCODE><space><KEYWORD>. For example, ABC YES.
• To let you know that the number you have replied to cannot accept "conversational text messages" and will only receive messages that use the above keyword format.