try with below code , i have added log as well. so check the log if it does not work, and let me know the log result as well
var mrvs = JSON.parse(producer.journal_entry_line_items);
var rowCount = mrvs.length;
gs.log('Length is '+ rowCount);
for (var i = 0; i < rowCount; i++) {
var gr = new GlideRecord('x_journal_entr_journal_entry');
gr.initialize();
gr.setValue('u_account', row.u_account);
gr.setValue('u_cost_center', row.u_cost_center);
gr.insert();
}
