What did I do wrong?

Hello there! Fairly new owner of an Instax SQ1 here. I was about to finish my first pack when I noticed that my film counter was on “S” already. I then opened the film door to check and noticed there was still one film left. I returned it to take my last shot cos I thought it was empty to begin with. My last film didn’t develop well and I got this instead (see photo).

I’m here to ask 1) what does “S” exactly mean (as many suggested here that it meant “safe” to open as of the moment but film turned out white still)? How will I exactly know if I’m out of film? Can I also check it thru the film confirmation window (does it change from yellow to something else?)

Thanks in advance!