I just tested this, and it's working for me.
Question 1: What OS and version are you on? I'm on Windows 11.
Question 2: Are you using the latest version from github posted on the link the first post? V1.4?
https://github.com/Imagix/uvtt2fgu/releases
Question 3: In Dungeondraft, are you exporting your map to vtt? Is the file extension .dd2vtt?
Question 4: Are you dragging the [mapname].dd2vtt onto the exe?
Question 5: Are they in the same folder? (I usually have a copy saved to a safe place, and then a copy in the folder where I'm saving my maps).