If you are having difficulties with spell check Mac in the new Office 365 suite, then it’s probably no surprise – especially if you’re not using Windows and not in the US. However I find that the advice provided on the MS website is actually working for me, so I’d suggest running through the first three steps below. And sometimes it has to be done on documents as you open them (every one). I was getting spell check errors for words like “advise” and “Australia”. Go figure.
If the steps below don’t work, you might need to try the other three methods (there are six in all) at the MS page on this issue.