It was fixed for most, I was able to do all task for him after they said it was fixed. Have you tried abandoning and redoing it? I think they gave coins as compensation for people who would have to re-do.
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[Resolved 11/4][Mercenaries] We are aware that players were getting stuck on certain Vol’jin Task levels. We’ve temporarily disabled him from visiting the Village while we investigate a solution. Update: This has been resolved via hotfixed. Impacted players will be compensated with 100 Vol’jin coins in the coming days, and Vol’jin has been reenabled as a village visitor.
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[Mercenaries] Fixed the issue where players could become stuck on certain Vol’jin task levels mentioned above. Impacted players will be compensated with 100 Vol’jin coins in the coming days, and Vol’jin will be reenabled as a village visitor. Update: Vol’jin has been reenabled as a village visitor.
Notes are from the Known Issues 21.6 thread.