/dev/net/tun is `character device` not file nor directory. Check it with `ls` command:
ls -lad /dev/net/tun
It shall look like (notice first `c`):
crw-rw-rw- 1 root root 10, 200 Feb 10 21:38 /dev/net/tun
To fix unload `tun` module:
rmmod tun
remove /dev/net/tun directory if it exist (directory is marked with `d` instead of `c`):
rmdir /dev/net/tun
Reload `tun` module:
modprobe tun
It shall create `character device` /dev/net/tun.