Normal maps are not colour maps but data maps so there’s data there that you corrupt when you try to paint it. They can be painted, it does “fix” issues but there’s usually a better way. Painting it is never 100% accurate and it takes longer.
To get the best normal map, I’ve found that using a cage gets the best results in almost every circumstance.
To make a cage, simply duplicate the retopologized character mesh. -> “Shift+D”
Rename it to something like “CAGE” -> “F2” This opens the renaming dialogue box in the viewport.
Go into the cage’s Edit Mode and press “Alt+S” to scale it along the normals. This needs to just be big enough to completely cover all parts of the high polygon model/sculpt. Be mindful not to change anything of the geometry or the cage will be invalid (no adding or deleting faces).
Then under the bake settings, turn on the cage checkbox and select the new duplicate.
The Max Ray Distance is now not necessary so it can be set to 0 if you like.
So the instructions again for creating the cage but simplified.
֍ Select the retopologized mesh.
֍ Shift+D (Duplicate it)
֍ F2, (Rename it)
֍ Tab to Edit Mode
֍ A (Select everything)
֍ Shift+A (Scale until it covers every bit of the high poly).
֍ Exit back into Object mode.
I hope that helps!