Itemprop error

We are aware of this and are working on it.

If you search the forums you’ll find lots of posts about this. That markup is correct as required by Google but the W3 Validator doesn’t like Schema.org markup. You have a choice, you can use the setting Use Schema.org Markup in All in One SEO Pack to disable it and ignore the fact that it is there because Google supports Schema.org, or you can leave it there and accept the fact that the W3 Validator will return errors.

where is the schema.org setting?

It’s under All in One SEO, General Settings – https://semperplugins.com/documentation/general-settings/#use-schema-markup