Hello @shozibul.islam95 ,
Generally in 3D you want quads or triangles. Quads are easier to turbosmooth, but can be a little less efficient for some things than triangles. Quads just about always look good when smoothed, but not so with triangles and N-Gons.
N-gons are faces that have more than 4 sides. These are problematic for a number of reasons, but the biggest is that 3dsmax (and other software) has trouble deciding exactly how normals should look in that case. So, they split the normals or make a feeble attempt to smooth it over, but it fails because well... N-Gons. Your turbosmooth gets especially confused and makes dimples when applied because there's no 'right' way to smooth some types of N-Gons.
When you shape merged these together, you created some N-Gons. It's nothing to freak out about, but it's the reason you have this problem. You can manually go in there and start cutting and connecting verts to fix it, but depending on the size of the model in your scene it may be a waste of time.
In that case you can try some sort of procedural normal smoother to see if it helps. The old school one is called "Smooth" in the modifier list, and you can set a angle and it will attempt to 'smooth' the normals for every face and it's neighbor provided that the angle between the centers of the faces is under that threshold. It's a good modifier, but it's dated and doesn't always give you want you want. (Sometimes, nothing does.)
Another modifier you can try is the WeightedNormals modifier. It can try to procedurally smooth out normal problems based on inputs and often does the trick, especially on really heavy models. Then there are external paid scrips such as Inverse Face Weighted Normals which also work on problems like this, most of the time.
Sometimes though you just gotta go in there and fix it manually.
But here's the thing... maybe you don't even need to do it this way. You can copy the model and delete everything around the leaf shape and just float that above the pre shapemerged base model if it's a decal or something. Or you can handle it in texture. But if it's a stepping stone to modeling more details, then the turbosmooth probably doesn't matter right now because there's a chance you'll need to retopologize the model anyways later on.
Make sense?
Best Regards,
