Change the drive letter in your Destination box to match the new drive letter Windows allocated the flash drive. Once it knows which drive you mean (as opposed to one that apparently doesn't exist), it can ask Windows what the LABEL & SERIAL for it are. You can then pick one (e.g. SERIAL) to 'remember' and it will then be able to figure out the drive letter for itself (via Windows, using that property you set), even if the drive letter changed in the meantime
Example: if you have Destination set in a profile as
Y:\BACKUPS
but now Windows assigned X: to the drive, and there is no drive Y, it has no way of polling for the SERIAL (etc) of the disk allocated to X: while you are telling it to use Y: - and obviously if Y is not allocated now, Windows can't provide us with details of the SERIAL (etc) of the disk allocated 'Y' - because there isn't one
- thus it doesn't list any.
Just change the drive letter and try again
Example: if you have Destination set in a profile as
Y:\BACKUPS
but now Windows assigned X: to the drive, and there is no drive Y, it has no way of polling for the SERIAL (etc) of the disk allocated to X: while you are telling it to use Y: - and obviously if Y is not allocated now, Windows can't provide us with details of the SERIAL (etc) of the disk allocated 'Y' - because there isn't one

Just change the drive letter and try again