Issue encrypting my Seagate drive with Disk Utility on macOS Sierra 10.12.6 I'm unable to encrypt an external hard drive because the encryption option seems to be missing. Various answers pointed to doing something with the 'Partition' button, but that button is greyed out. To solve the greyed-out issue, evidently, I first need to click on the 'view' button, but I have no view button! 

(Specifically, I don't see a view button on my screen and the partition tab is greyed out.)
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!