Thanks for reporting this and for sharing the screenshot. Lens Blur is a premium feature in Lightroom, so could you please confirm that Lightroom is signed in with the Adobe ID associated with your active subscription?
Also, please go to Lightroom > App Settings > About Lightroom and share a screenshot of that screen here. This will help us confirm the Lightroom version.
If you do have an active subscription, please also try signing out of Lightroom and signing back in with the Adobe ID that has the subscription, then test Lens Blur again.