Only certain files can be repaired, e.g. ones that were scanned with the VRDB process (a little like a snapshot of the file), otherwise avast doesn't know how to return them to a clean state.
Not all files are scanned by the VRDB process, mainly windows system files, .exe, .dll, etc. New files that have been added to the computer that have yet to be scanned by the VRDB aren't protected until the next scheduled VRDB.
So files that have been scanned by the VRDB process in general can be repaired.
Always start with the least damaging (to your system) action first when dealing with a virus, etc. - Repair - Move to Chest - Move - Delete (if all else fails). Once you have deleted something there is little that you can do after that especially if you chose to delete an infected file that the system needs. Best to ask here if in doubt.