If the Reading Pane setting is turned off in Outlook, the phish button will appear greyed out—here’s why.

The extension point for Outlook add-ins applies only when a message is in Read or Compose mode. When the Reading Pane is disabled, Outlook is technically in neither state, so the add‑in cannot load and therefore will not be available in this view.
This behaviour is documented in Microsoft’s guidance: Outlook add-ins overview.

To make sure the phish button is active, you will need to enable the Reading Pane in Outlook, as shown below.