Capture encrypt 802.11 and decrypt later?

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For general 802.11 decryption processes, see the wiki page here.

In particular, there is a Gotchas section which could be useful - make sure you take care to address those concerns. With no other information that what is presented, my first guess is that you don't have all four EAPOL frames from the client and the network under review is WPA2. If this is really the case, you don't have sufficient information to be able to decrypt.

You do not have to be connected to that network in any way - a pure passive capture from an otherwise unused WiFi adapter on that channel that can pick up the client/AP traffic can be sufficient.