cmalco
Make sure your arc is tangent to a horizontal line extending from the top of your cylindrical boss, or above that line. Else, it will fail.
thank you dude yeah IT WORKS I COINCIDED B4 by creating an intersection point , after creating a reference line and making in tangential getting the profile
Yes the 'rib' feature is a bit finicky about that. I've had designs where I needed the arc tangency below the point of coincidence in which I resorted to cutting it with a surface. Such is the CAD part of design... always with a new challenge to get the desired result.