I think that currently (5.1) TaskService does not support JTA transactions, there were number of posts on the forum discussing that and even some guys have made some implementation to make that working.
You can change your persistence configuration to be non JTA, that includes transaction-type attribute of persistence unit and data source tag. Transaction type should be resource local and data source should be non-jta-data-source. That should make task service available and work properly.
HTH