I wanted to know how do you get incompatible lightning bolt?

You would get the incompatible lightning bolt for another sim if they have the quality that your sim finds unattractive. When you look at your sims history, personality, memories, interests, there should be a little sub tab with the lightning bolt, and that will have the 2 qualities your sim thinks are hot, and the 1 quality they think is gross. For example, blonde hair and glasses wearing is hot, but fatness isn't.
The fat sims will get incompatible lightning bolts from your sim for sure. And sometimes other sims are unattractive to your sim for seemingly no reason at all, and you'll get an incompatible lightning bolt.

Hope this helps! Happy Sims playing :-)