Sage Line 50 Export not working with following error message:

The full error message is:

Export Results Outcome
There are no Client Result to be exported.
Only Clients who have an invoice to be exported, have no Sage Line 50 Ref number and have not been exported previously will be exported.
There are no Invoice Result to be exported.
Please note you can only export invoices without errors.

Please check that all invoice records have a currency ID.