When you see that message it's usually because one of your configured repositories are (temporarily) offline.
What you do is to (in application manager) select Tools -> Log, and check which repo failed (look for errors, not warnings), when you have identified it you go to application catalog, tick 'details' on each of them until you find the right one, then tick 'disabled' (or 'deactivate'). After that it'll work. At some later stage you can try to re-activate to see if the repo came online again.