A task will be considered 'claimable' only if the Candidate User or Candidate Group of that particular task is specified.
In this case, you have logged in as admin, so setting the Candidate User for the HelloWorld task as 'admin' should set the task as claimable. Any other users that you have defined in the BPS Server should also count as Candidate Users.
You can also have a Claimable task assigned to an entire group of users, by specifying the User Role. I have defined user roles like 'employee' and 'manager', so setting the Candidate Group as 'employee' will set the task as claimable for all users with the 'employee' role. It should look like this