Welcome to the Asana Community Forum @Jon_Paul_Ashworth 
Sorry to hear about the issues you are facing.
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Can you check if the app has fully synced all changes? Like see here often some changes are pending sync.
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Does the same problem appear on the Asana web version too or only desktop?
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Do you have any filter or sorting enabled under my tasks? Because this could be one reason on why it won‘t let you add a section.