You're getting a list of items and then turning the list into a string, which
is causing problems. You need to loop through that list and apply your name
change to each item. Also, getting every item
will include disks, which may
not be what you want. You can also simplify a bit. Here's how you could do it if
you don't want to change disk names:
tell application "Finder"
repeat with finderObj in (items in desktop where class of it is not disk)
-- Make whatever change you want to the name here
set the name of finderObj to the name of finderObj & " test"
end repeat
end tell