clash detection

Assuming that's a default mitered connection and not s true overlap, there shouldn't be a clash.   Does your Rules tab have the default suppression rules checked similar to the attached image?

yes it is similar , and still I got clash

OK, they do appear to be identical.

I would first suggest using the Connect Forms As L (Bisector) tool on those two walls, just to be sure that the joint does not contain any overlapping geometry.   Based on the image alone that's the only thing I can think of.

i used that one to solve the problem but it is not the default one , how I can make it default

But that is not a viable solution as a mitered cut is proper for unification and centerlines.

I don't understand.... the two walls in the image already have a Bisector connection.  By using the Connect tool after the fact, you are just insuring that the connection is valid.   What I am not sure of though, is why the connection would not already been in this state.

because of the comment earlier I am wanting to make sure we are not having to replace the miter connection with the L connections as they do different things when rendered

because of the comment earlier I am wanting to make sure we are not having to replace the miter connection with the L connections as they do different things when rendered

Correct, Bisector refers specifically to a mitered connection while Parallel is a typical "L" connection.  

Though I'm still unsure why the original Bisector connection was not valid - it seems that was the cause of the clash, as though there was a true overlap and not purely coincident faces (which the suppression rules would cover).