Did the school explain
why you were ineligible for an ATAR? Sounds a little suspicious if the sheet was the only thing saying you couldn’t, and that might indicate it was a clerical mistake.
I have limited knowledge of SACE requirements for completion, but my 2 mins of research dug
this up, which is a link of the requirements to successfully get your SACE. I couldn’t actually find anything about how to get an ATAR in SA, so I’m not sure, but the link above suggests that you need an English subject/ course in either Stage 1 (Year 11) or 2 (Year 12), as well as a few other requirements, to successfully obtain your SACE. I assume successful completion for the SACE would probably be involved in getting an ATAR, in addition to
these requirements, to get into uni. As I said, I’m not well versed in the SA version of things (I’m Victorian), so my answer is likely wrong. Perhaps you could contact someone about this, e.g. another knowledgeable teacher or SACE Board?