Create expense report on an Open task
Question
Why can I not choose an open task in the expense screens?
Answer
The issue with open tasks is, as long as a resource has not submitted any time against it, there is no task assignment. This is to protect the database from being flooded with 'empty' assignments - consider a company with 1000 employees and 100 open tasks would create 100.000 assignments, maybe many of them unused.
Check in the project edit screen for the setting 'Allow users with an assignment to book expenses'. So as long as there is no time there is no assignment and the task doesn't show up in the expense section.
If resources that cannot book time on a task have to submit expenses, you can add them in project as expense-only user. Then the task will show up in the expense section, with or without task assignment for that resource.