Issue encrypting my Seagate drive with Disk Utility on macOS Sierra 10.12.6
You will not be able to do this if the disk is formatted as FAT, FAT32, or EXFAT. In order to encrypt the disk, it has to be HFS or APFS.

You will need to format the disk to HFS or APFS and then you can encrypt. When formatting, there should be an option to format as APFS Encrypted or macOS Extended Encrypted. You can select either one of those or you can encrypt after formatting.

Good luck!