Not enough bones - you have no hip bones to fix the hip area, so it will probably always do this.
Not enough, or too many vertices in the mesh, I cannot see your blend file from my machine as you have not posted it, therefore I can’t tell if this is the case…
You should study the Rigify or Flexrig armatures to see the best method of rigging bipeds. Put the armature right first, then look at the mesh, then parent with Automatic Weights before doing any Weight Painting.