You are seeing this message because when the device was encrypted the Full Disk Encryption method was chosen.  ESET Endpoint Encryption 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 ESET Endpoint Encryption 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?