Hello, SONGNGANG
I think you need to set Angle from Type Parameter to Instance Parameter as screenshot below. If it's a type parameter, the angle is always fixed. If it's an instance parameter, it can adopt to the angle between the two connecting pipes if you set the constraint correctly.
You can refer to the out-of-box content Libraries\US Imperial\Pipe\Fittings\Generic\Elbow - Generic.rfa

Thanks,
Zhengrong
Zhengrong Fu
Product Owner