You can't save the record because it contains a Foreign Key value to a different table, and there is no matching record in the other table.
This often happens when you do the inserts in the wrong order - suppose you have a new customer and you want to record the sale.
You have two tables:
Customers
ID int, IDENTITY
CustName NVARCHAR(256)
And Sales
ID int, IDENTITY
Item NVARCHAR(256)
Price decimal
CustID int, Foreign key to Customers.ID
You can't insert a row in the Sales table first, because you don't have a customer to reference it to - you may create the new Customer row in order to get the ID value you need to insert in the Sales table rows.
In your case, try creating the Bills table entry first, and then you will have the ID value you need to insert the Sales rows.