Looking at the release notes (https://docs.mendix.com/releasenotes/studio-pro/8.0), it seems that mendix assist is supported from beta 2. So I guess you need to update your modeler.
Hi Biao, yes Johan is correct. Mendix Assist features for Studio Pro became available with Mendix Studio Pro 8 Beta 2 and at the moment of writing Studio Pro 8 is already out of Beta, so you can get the GA (general availability) version.
You can refer to the release notes for more information: