According to man page, the -f option has this goal:
man
-f
-f, --force do not prompt before overwriting
I found the reason. It was due to permissions.
I have updated the directory permissions as chmod a+w <directory>
chmod a+w <directory>
Other possible solutions are adding a rm -f targetfile before the mv command.
rm -f targetfile
Ofcourse GHugo was correct to add -f option