You must select a Legal Entity in order to search for matched transactions.

The Legal Entity Organizations that may appear in the drop-down list will include only those organizations that have been defined to be a "Legal Entity".  This relationship is defined on the Organization >> Edit >> Financials tab (Financial Organization Type >> This Organization is a Legal Entity radio button field).  Inactive Legal Entities are not included. Depending on the user's role, Organization Access restrictions may limit which Legal Entities are visible.