I haven't done this quest, but make sure you have the right type of slab, does it say smooth quartz or any other type, or is it even slabs? Try to check everything about the quest to make sure you haven't done it wrong. If not, you can report it as a bug here: https://reports.cubecraft.net/bug_report
Hi! This issue is already known within the staff team (: it’s a bug that has been happening for quite some time now, but know that we are aware of it! I will lock this thread since the question got resolved!