During normal operation, this function should remain enabled as it is the only way to receive information about critical operations. During a presentation, however, this would only result in interferences. This is why the display of error messages can be suppressed and restricted to the absolutely necessary. Nevertheless all messages will still be recorded in the Log file. See Displaying log information.
In the toolbar click on 
Enable all warnings.
As an alternative, you can also click Extras followed by selecting Enable all warnings.
For permanent installations, the warning messages can be permanently disabled via Options - Global Options - Program start.
Please note that the warnings are enabled when the symbol is depressed (default setting) and can be disabled by clicking it.
See also