The current keyboard navigation plugin requires the use of blockly v12 and above. There are many new features in v12 that are required in order to support keyboard navigation.
There was an older version of keyboard navigation (technically in beta, as it was v0.x) that has many limitations and is not fully accessible. Version
https://www.npmjs.com/package/@blockly/keyboard-navigation/v/0.3.18 is compatible with blockly v9 but note that its functionality is quite limited (not all elements are keyboard-navigable, there are not many keyboard shortcuts, no way to open the context menu with keyboard, and many other limitations). We are unable to fix any bugs in that version or provide additional support for it. We would strongly encourage you to upgrade to the latest version of Blockly. Please let us know if there are particular features that are preventing you from doing so.