Once an objective is complete, the last step is to Validate the objective which allows you to agree or disagree with the individual's supporting completion statement. If Validation doesn't appear on the right-hand actions panel the objective may need to be approved or is still awaiting completion.
Check the Objective Status on the right-hand side to see what's missing. Remember only a Manager can validate an objective.
You can read more on the states and status' of objectives in our article Objective Permissions and Rights