It is more sensible to do the appendix first, with all the workings, and then write the report, and that is no problem.
Just make sure you head up the pages with ‘appendix’ or ‘report’ as relevant. (And if you later find you need more workings and haven’t left enough room, then it is no problem having appendix, then report, then appendix, provided again that you head up the pages so that it is clear to the examiner. (And if you have more than one page of appendix then head them appendix 1, appendix 2 etc.. Same for the report if you use more than one page.)
All the matters is that it is all clear to the marker – the order of the pages is irrelevant.