Hi,
you have to configure channels/link aggregates identically on both sides.
If you use LACP at one end, you have to use LACP on the other end also.
To be sure, that lacp negotiation starts, I prefere to set both side to active, allthoug passive should work, if APs send LACP packets.
After configuring the channels and connecting the APs you should prove consictency e.g. by issuing show commonds (show lacp, show logg and likewise).
E.g. all members should be selected. Non standby or errordisabled.
On Ciscoes you might have to configure channel-protocol LACP. Cisco also uses PAgP as a channel control protocol. But mode passive/active should select LACP automatically nowadays...
Just to be sure:
Did you check, that the errormessage is truely related to the two portchannel member ports? If client roams from one AP to another, this can also generate MAC flaps.
Regards, Jörg