Final answer:
Upon receiving the lineup with only one Libero designated, the official should proceed with the game as usual unless the coach indicates an error which then must be corrected before the match starts.
Step-by-step explanation:
When the coach from Team A submits the lineup for the first set with #9 designated as the Libero, and leaving the second Libero box empty, as a referee or official, it is important to address the issue. In volleyball, teams may designate up to two Liberos before the match starts, though it is not mandatory to have two. If only one Libero is listed, then proceed with the game as usual. If there was supposed to be a second Libero and it was an accidental omission, the coach would need to correct this before the match begins. Otherwise, the team will continue with a single Libero for the duration of the match.