Boolean union

boolean.3dm (1.6 MB)
hi this is nice forum , and i am newbie still, want to ask how can i boolean union these in my 3dm please, any reason ?
big thanks

Your two shapes overlap by two tiny amounts AND one of the overlaps is not a closed section. They also alternate between overlap / not overlap / overlap, which is not very clean either.

Try Curve → Curve From Objects → Intersection and examine the two resulting curves that show up.

I did a video on trouble-shooting booleans that might be helpful. It’s free, but it’s part of a Rhino 5 course at that requires a subscription.