The IPB version installed is the latest. I've installed several widgets from the marketplace and see that some of them are not showing even if  I configure those correctly, Is there a way to see the actual php error instead of the standard text. this widget can not be shown. ?

Thanks

That text doesn't always mean there's an error - sometimes widgets don't show based on configuration settings for example. I would recommend editing the widget after embedding it into the page to see if there are any configuration options that might explain why they're not showing, and if you don't see anything specific, contact the author of the widget for assistance.