The Counterparty drop-down list in the Settlement (Invoice) Details form displays all organizations you have saved in Settings > Connections. Having your connections saved here is important so you can quickly select them when creating contracts, deliveries, or in this case, creating a settlement or invoice.

If the drop-down list is empty or incomplete, make sure that the connection you are looking for is saved in Settings > Connections. To add a new connection, see Add companies and contacts you buy, sell, store, and farm with.