That the message of the diagnostic session disappears is normal.... Strange that he offers to connect to nac and shows current calibration....
Btw:
I had issues using windows 11 and the com ports. Is there any chance for you to use windows 10 instead? For me this solved the problem connecting via the tool.
All sketches and the tool were downloaded directly via the links in the tutorial at vlud's webpage. Only thing I needed to change in sketch was setting to 8mhz...