Hello @ WangShermaine-9311
If I go to disk management, I can add a drive letter and it will be displayed as a drive in my computer, but when I log off and login again, the drive missing again.
Yes, this is the default behavior.
Attached virtual hard disks will always be detached when computer will be shut down (or restarted), and need to be re-attached after every boot.
You could create a task to permanently attach a VHD, see How to open, unmount, or permanently attach a VHD file in Windows 10 - Digital Citizen
or you could use FSLogix Disk Management Utility :
https://learn.microsoft.com/en-us/fslogix/disk-management-utility-reference
Best Regards
Karlie