I tend to forget that not everyone has upgraded theme before, sorry.
For anyone else if this happens - you may have to select another theme - 2013 or something, delete symbiostock, install symbiostock , reinstall child theme (do not delete child theme)
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Regarding the number of sub-categories. I have no idea how many is the limit, I have quite a few
. The number of main headings is limited by the space available, if you have too many they will spill over onto the next line.