Since you are new, i'll explain a bit. Windows 98 is an 'old' version of Microsoft Windows. Memory sticks/flash drives can work on Windows 98 with a driver that you have to install, and the driver makes the flash drive run on your computer.
Try reinstalling the drivers for the flash drive, that might work.
It will also be helpful if you tell us information about the flash drive. What brand it is, what model, what company, how old it is, ect...