If the Reading Pane setting is turned off in Outlook, the phish button will be greyed out; here's why.

The extension point for messages is either Read or Compose. When the Reading Pane is off, you are doing neither, so the add-in won't be available in this view. This is documented in the following Microsoft article: Outlook add-ins overview

To prevent the button from being greyed out, you need to enable the Reading Pane within Outlook as shown below.