Entering the interior of a structure

I created a mesh with a doorway and a ramp leading to the top. However, I’m blocked from entering the doorway. Here’s a screenshot:

My first guess would be that you had “automatically create collision mesh” (or something like that) ticked when you imported the fbx file. Open your mesh properties in the content browser and check the “simple” collision. - You can also check collision in the viewport by pressing Alt+C.

the 2nd guess is that the player characters collision is too large. They often have a quite big capsule around them.

Maybe check the collision tutorials as well: