The Microsoft Access database engine cannot find a record in the table <name> with key matching field(s) <name>. (Error 3101)

In a one-to-many relationship, you entered data on the "many" side for which there is no matching record on the "one" side. For example, this error occurs if you join a Customers table and Orders table on a CustomerID field, and then add an order using a CustomerID that does not exist in the Customers table.