Can you post a screen of what you are talking about?
I can't really decipher what the problem is, if you are having a hard time do to heights, or object usage or what. A screen would really help out.
Even if the object is "walkable", it will still generate red on the impass overlay.
If your bridge is too high it will glitch out (this has not been confirmed on CoH 2, this was a bug in vCoH), so maybe try height adjustment.
Other than that it could just be the model you are using isn't coded properly. As you spend more time in the editor you will notice more and more things that simply don't work.
If none of this helps you, post that screen and I will see if I can try to help further.
The screenshots are linked in my post. Thanks for the help