How do I permanently solve the message on my S60 which reads 'Anti-skid service required'?

There are several parts that could cause that message, and getting it to a garage to properly diagnose it would be a great idea. If it's the I think it was 2001 to 2009 model, one huge culprit for that error, especially if it is intermittent and more often seen in hotter weather, is the "clockspring" or steering wheel position sensor. It determines the position of the steering wheel to help the anti-skid system determine how to apply breaking or torque to the wheels as needed. It's a known failure point and easy to replace, but can cost between about $250 and $350 easily. Other culprits could be a yaw rate sensor or a sensor in the wheel area that determines the rotation speed of the wheel. That part is super easy to get to and cheap to replace, but usually don't go bad.