Why can't mount /mnt on /dev/sda2?

Insert usb in the router,connect to my router:

ssh root@192.168.8.1

List all partitions on sda:

root@GL-A1300:~# fdisk   /dev/sda
Device     Boot   Start      End  Sectors  Size Id Type
/dev/sda1          2048  1230847  1228800  600M 82 Linux swap / Solaris
/dev/sda2       1230848 31948799 30717952 14.6G 83 Linux

I want to mount all content in /mnt to/dev/sda2

root@GL-A1300:~# mount /dev/sda2 /mnt
Mount is denied because the NTFS volume is already exclusively opened.
The volume may be already mounted, or another software may use it which
could be identified for example by the help of the 'fuser' command.
mount: mounting /dev/sda2 on /mnt failed: Resource busy

Is there on way to mount it?