How do I erase everything inside of a static mesh box i created around a component of my world mesh ?

Unreal 5.3.2. I’m trying to erase vehicles from my mesh. I am creating a box mesh around said vehicle, and trying to delete the contents. It only deletes the box when i push delete. And it will not let me use mesh knife for some reason. I checked and have all plug-ins active. Any thoughts?
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