The error message is pretty clear:
The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Installments_Loan". The conflict occurred in database, table "dbo.Loan", column 'Loan_id'.C#
You have a rellationship set up between the two tables, which requires a value in another table in order to save this one. Look at your relationships - it shows the name in the error - and create the other table entry first, or alter the relationship.