I tried to surf the forum but still have no idea what happened.
I have removed all skins, animations, all other bone and constraints but it still show a warning mark and the IK is not working
Warning mark on IK constraint
The warning mark on an IK constraint typically indicates a problem with the constraint's setup in relation to skins. According to the Spine User Guide, a warning is shown on a constraint's tree icon if the constraint's target bone is in a skin, but the constraint itself is not in that skin, or if all of the constraint's constrained bones are in skins, but the constraint is not in any of those skins. To resolve this, ensure that the constraint is added to the same skins as the target bone. Additionally, if all constrained bones are in skins, make sure the constraint is included in at least one of those skins.
It took half hour die trying surfing internet to search for a solution, the found it myself right after posting to forum. Silly me somehow I uncheck scale on IK bone. 
Turn it on and constraint worked like normal
raaan127 Sorry for the confusion! I thought it was hard to understand why this warning was displayed, so I created an issue to address it here: EsotericSoftware/spine-editor905