Your code expands as:
cp myfile /home/user1/Desktop/ /home/user2/Desktop/ /home/user3/Desktop/ /home/user4/Desktop/ ... /home/userN/Desktop/
From man of cp:
Copy SOURCE to DEST, or multiple SOURCE(s) to DIRECTORY.
which means, it will try to copy "myfile /home/user1/Desktop/ /home/user2/Desktop/ /home/user3/Desktop/ /home/user4/Desktop/ ..." to /home/userN/Desktop/
it will throw an error because except "myfile" all other where directories...
try this,
UserS=($(awk -F':' '{ if ( $3 >= 500 ) print $1 }' /etc/passwd))
for i in ${UserS[@]}
do
[ -d /home/"$i"/Desktop ] && cp myfile /home/"$i"/Desktop
done
