Haven´t built the Sturmtiger, but I have built the bergetiger, which also comes with link and length. And knowing Italeri they are probably the same molding. There were approx. 10 single links left over when I was finished. I used 7 of them as turret armor as an extra. If there is a gap, just place in one extra link. From what I remember it gives the amount of tracks to go arounf the sprocket ... maybe you started further back than you were supposed to. If that is the case .. use an extra single track at the top and cut one off from the length at the bottom.
This is just a stab in the dark ... hopefully we´re on the same wavelength. Maybe its an idea to post a picture of your set up if the above does not help.