Hi, @JabbaKITE you might wanna follow those quickstart guides I posted a link to above, there the basics are explained step by step.
Basically for your AI to move on the navmesh, you need to have a navmesh, the AI needs to be possessed by an AI controller, and you would have to call a “Move To” node and then the AI needs to have a valid way to the target location.
For debug I find it useful to have one AI in the level and then watch the behavior tree (the variables and the execution flow) while the game is running (so you would need to run the game in PIE, not standalone).
Also you can visualize the area the navmesh is covering by pressing P Navmesh Content Examples | Unreal Engine Documentation