A referential integrity constraint violation occurred

We can't solve this for you: it's got too much to do with your data, and how you are manipulating it.

What's happened is that a value is left in a table, or inserted into a table which depends on other table information, or which other tables depend on.

For example, if you have an Invoices table, and an InvoiceLines table, then the InvoiceLines table will have a InvoiceID column which refers back to the Invoices table and ties all the data together. If you delete a row from Invoices and leave the ID's in InvoiceLines then that causes a referential integrity failure, because there are lines who no longer have an invoice. Similarly, if you ad lines to InvoiceLines without creating a matching Invoice row first, you have the same problem.

But we have no access to your data, or to your user input - so we can't do anything to work out why this occurred!

So, it's going to be up to you.
Fortunately, you have a tool available to you which will help you find out what is going on: the debugger. How you use it depends on your compiler system, but a quick Google for the name of your IDE and "debugger" should give you the info you need.

Put a breakpoint on the first line in the function, and run your code through the debugger. Then look at your code, and at your data and work out what should happen manually. Then single step each line checking that what you expected to happen is exactly what did. When it isn't, that's when you have a problem, and you can back-track (or run it again and look more closely) to find out why.

Sorry, but we can't do that for you - time for you to learn a new (and very, very useful) skill: debugging!