The "Approve Selected" button is not enabled (greyed out) when selecting all the approval tasks to approve. This issue only happen if the approver is set as Group. If approver is set as user this issue does not happen.
Select user as Approver when creating workflow.
This Product Defect (TF00352069) has been fixed in the latest available cumulative patch, you can get more information about it here:
ActiveRoles 6.9 Generic Patch 3 (133478)