OpenERP is asking you to fill the field "Analytic Journal" on the Journal you're using on the expense, (field "Force Journal" under the "Accounting" tab or, if empty, the journal specified on your employee's form).

Search that Journal under Accounting > Journals > Journals (or just open the related record from the employee or expense form) and then, from the analytic journal field create a new analytic journal of type expense. give it the name and the code you prefer.

save everything and you should now be able to validate your expense.