A yellow warning will not prevent you from saving the listing. It just alerts you to a possible issue with the listing (ex: there is another listing for that address that is currently on the MLS).
Click Submit Listing again and Matrix will accept it.
A red error, however, will prevent you from submitting the listing. This means that there is at least one field that requires fixing before you can submit the listing. There will be a link you can click on that will tell you exactly what field(s) is problematic.