You are seeing this message because when the device was encrypted the Full Disk Encryption method was chosen.  DESlock+ Go only works with removable media using the File mode.

Please see this article for details of the different methods of encryption: KB322 - What are the removable media encryption modes - (file / full disk)?

If you wish to change the disk from using Full Disk mode to File Mode so you can use the DESlock+ Go feature you will need to decrypt then encrypt the device using file mode as detailed in the articles linked below:

KB324 - How do I decrypt a memory stick or external disk?

KB323 - How do I encrypt a memory stick or external disk?