The account number 9999/999 is reserved for cash transactions in the Invoicing & Billing (I&B) module and therefore cannot be used as a ledger or debtor in Accounts. Additionally, you should not create a customer profile using 9999/999 in I&B, as it is designated as a special account number. To fix this:
Delete 9999/999 ledger from debtor maintenance and the chart of accounts.