Quests&Objectives
Solo Multiplayer
BP C++

Start and complete

Get the Quest Component from that player's PlayerState. Category Quest.

ActionNode
StartStartQuest or StartQuestById
Advance an objectiveCompleteObjective
Fail via a fail idFailObjective
Fail one questFailQuest / FailQuestById
AbandonAbandonQuest / AbandonQuestById
The same Start / Complete nodes as solo. On a client they send a Server RPC.
Fail Objective. This call fails every Active quest in the current phase that uses that Fail Id.

Do not use the bool return to update client UI. On a client, the function returns false after it sends the RPC. Read events and GetQuestsByStatus instead.

On the server, true means the change applied.

CompleteObjective still matches every Active quest in the current phase. That is the same as solo.