I am trying to delete a Supplier record but I am getting an error message. Why is this?

You will not be able to delete a supplier record that has been used, i.e. the supplier is associated to a candidate.

The process would be to remove the supplier reference from any associated transactions.

If you have produced invoices for this supplier you would need to remove them/void them, cancel the timesheets, change the assignment and change the candidate record then the system may allow for the supplier record to be deleted.