The Meta Keywords features are disabled by default in All in One SEO. You can buy AIOSEO Pro for more features.
you can enable these features by clicking on Search Appearance in the All in One SEO menu and then going to the Ydvanced tab and setting Use Meta Keywords to Yes.
This will enable the Keywords field in the AIOSEO Settings on the Edit screen for your content. You can add meta keywords to any content such as a post, page, category, tag, etc. by editing the content, and clicking on the Advanced tab in the AIOSEO Settings section.