Problems when encrypting Time Machine

Whenever I try an set up Time Machine on my USB drive, and select the option for encryption, I get the following error message. I do not get the message when I deselect encryption, and Time Machine can be set up on the same drive.

Can anyone say what the problem is?

I believe you just need to follow these steps:

1.Open Time Machine preferences.
2.Click Select Disk or Add or Remove Backup Disk (if you have multiple backup disks).
3.Select your backup disk, then click Remove Disk.
4.Now set up the disk again as an encrypted backup disk. For instructions, see: Choose a backup disk and set encryption options.

A more detailed description of the process can be found here;
https://support.apple.com/kb/PH14110?locale=en_US

However why would you want to encrypt your backup? Is your Hard Drive encrypted as well? I ask this because if you are in a state of duress (which most of us are when our HD crashes) the last thing you need is difficulty unlocking your backup. eg

"To encrypt the Time Machine backup, you must provide a backup password. No one but you should have access to your password. Choose a hint that helps you remember your password. To restore files from encrypted Time Machine backups, you must provide your backup password". https://support.apple.com/kb/PH18858?locale=en_US

Now where did I write that password? Hopefully not just in your computer keychain.

Yes, I have File Vault enabled, so I thought I should also turn on TM encryption also. I will not forget my password for TM backup.

I tried your steps several times, with no luck, although they have worked now for some reason. Thank you

I'm glad it worked for you albeit not the first time. I ascribe to the keep it simple system so I suppose having seen many people have problems with encryption, not because of the encryption itself but because of complications caused when attempting to repair other issues. So unless you have a lot of sensitive information stored on your computer I cannot see the need nor does it protect you from many online scams one of the primary treats to identity security. At the end of the day your privacy/security is only as safe as your login password.

I've turned off File Vault, I was getting much slower read/write speeds with it turned on. They went down from 330mb/s to 200mb/s. I don't have sensitive data on my Mac, I had heard that if you're on an open wifi network, people can intercept data from you?