Boolean fail when intersect

Hi! I’m new to 3d modelling here. I’m trying to intersect 2 bodies which seen as intersectable but when I select them it shows ‘the boolean failed.’ what does it mean or how do I fix it?

Can you post a screenshot?

I started over and now it works. Switched from loft to drag the design to create bodies in some parts. I might have selected something wrong during Loft it last night. Sorry for bother. Thanks.