I'm 99.9% sure you cannot do family plans via the App store.
You need to do so online here by following: https://help.dropbox.com/accounts-billing/plans-upgrades/dropbox-family-faqs
Edit: yes you cannot use mobile devices to get Family Plan. You need to cancel that plan and rejoin via the website: https://help.dropbox.com/accounts-billing/plans-upgrades/troubleshoot-family-plan
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