Color banding is generally a thing that happens with digitalart, mostly when painting large gradients without texture especially with the round brush or airbrush.
An easy way to fix it is to duplicate the layer or to select affected area and „add layer via copy“. Then add surface blur to it until most of the banding dissapears. Then you can add a mask, paint with black, to remove it from areas you don't want the blur, and then if needed, add a new layer to paint back in any details that might have been lost.
Another/additional way already mentioned is to add a new layer on top, fill with 50% grey, set to overlay, and add noise until banding mostly dissapears, then add mask again to remove from unwanted areas, and adjust layer opacity if needed.
Next way is to add any grey textured layer on top, set to overlay and adjust opacity, erase, adjust or add mask as needed.
Final and most time consuming way is to paint over affected area with smaller or more texture brushes.
And to generally prevent it from happening again, you can either avoid painting gradients with very large brushes, and paint in with smaller strokes instead, or use more textured brushes.
Hope that helps