The question is about 3 problems. Exactly what error message are you getting?
No app to open something is available? You have to install an app that can handle that file type.
A URL is unavailable? Then it's unavailable - there's a temporary problem, the site is no longer there, something that you can't do anything about).
No app is available to open a URL? If the URL starts with http:// or https://, you have no web browser. If it starts with something something else, you'll have to tell us what.