Here is the corrected script[1]:
tell application "Finder"
set theFile to selection
open theFile using application file "QuickTime Player" of folder "Applications" of startup disk
end tell
This can be simplified by specifying the application by bundle ID instead of location, in which case Finder will find it for you:
open theFile using application file id "com.apple.QuickTimePlayerX"
Or, if you want to do stuff with the file(s) once they’re opened:
tell application "Finder"
set theFiles to selection as alias list
end tell
if theFiles is {} then error "No files selected."
tell application "QuickTime Player"
activate -- (ensures any "can't open file" error dialogs are visible)
open theFiles
-- do other stuff here
end tell
--
[1] The syntax error is due to the special behavior of with
/without
parameter labels, which expect to be followed by a single keyword (or comma-separated keywords). AppleScript reads the command as open theFile using application
—and since a command can’t be immediately followed by an expression, in this case a literal string, it reports a (frustratingly unhelpful) syntax error.