Hi Dimitris, and welcome!
I’ve just had a look at your file. In my opinion, there’s nothing major wrong with it. I think the softness is a combination of lens blur (it is a basic lens) and also some subject movement. Have a look at my jpg. Any better? 
I’ve applied a preset I have for the diffuse or sharpen module, which gives a fairly aggressive sharpening, and I think it helps. You can download the .xmp file and load it as sidecar in dt’s lighttable view if you want to see what I’ve done.
I’d suggest trying a few experiments, with a stationary subject, as has been suggested already. Try different apertures, VR on and off and anything else you want to try.
Oh, and a better lens, especially a prime, would be better…
Good luck!
Edit: I’ve just looked again, and I think I may have misused diffuse and sharpen a bit. I do think that there’s definitely motion blur there - maybe there was some camera shake?
_DSC0731(1).NEF.xmp (8.0 KB)