Option 1 – If the option to encrypt a USB drive is missing from the windows explorer context menu you might need to reformat the USB drive (without selecting quick format).
Full format a USB drive
Option 2 –
If this does not work use the command line for DISKPART to delete the partition. Once the partition is removed use disk management to create a new partition. Eject and re-insert the USB drive (dont forget this part!) and Bitlocker should be available once more.
Drive Tools – Bitlocker