Thanks for reaching out. Hope you don’t mind us responding in English, as this is the official language of the Community. If you've cancelled your Duo plan and are now trying to join the other person's Duo, then most likely your own subscription is still active and that's why it doesn't work. When you have your own Duo or Family plan, you have to wait for your account to revert to free after cancelling in order to be able to join another plan.
Hope this clears things up.
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