Solution
To lock the TestStand sequence file, you may go to TestStand Edit >> Sequence File Properties to launch the Properties dialog box.
Go to the Advanced tab, and you could choose the Password Protection to be Disabled, Not Editable and Not Viewable.
If you choose Not Editable or Not Viewable, you may set the password for the sequence file.
Please note that once you lost the password, the NI team was unable to help you retrieve the password or the content of the sequence file. Thus, do remember the password that you have set for the particular sequence file.