Hi Jing,

If there are no other users in the data, try exiting Ledger and rebooting your workstation. Then try again with the data backup.

Are you in multiple copies of Ledger, or in any of the other programs such as AR or AP? That would also block the backup.

It could also be the server not releasing a particular data file. If it were this scenario, a reboot of the server would be necessary.