The Chrome operating system is, by its nature, vastly more secure than Windows. Windows relies on a central 'registry' which links everything together on a computer, so it's relatively easy for a virus (or other malware) to spread across all parts of the system. Chrome, like Android, 'sandboxes' each part of a computer's operations, so that they all function entirely independently of each other:
https://support.google.com/chromebook/answer/3438631?hl=en-GB
So Chromebooks don't need any anti-virus software to be installed; the Chrome operating system is effectively impregnable to outside attacks anyway. If you want to increase your privacy though then some of the suggestions here might be relevant to you:
https://www.pcworld.com/article/3064762/security/how-to-configure-your-chromebook-for-ultimate-security.html