The Trade Debtors account has a negative balance
You probably specified Trade Debtors as the account to receive customer payments and then recorded some sales. To ensure that all of your sales receipts are credited correctly, you should allocate them to an income account, not to Trade Debtors.
If you accidentally allocate your sales to Trade Debtors, MYOB Accounting Plus both debits and credits your Trade Debtors account whenever you create a new invoice, and nothing is posted to your Income account. Then, when you receive customer payments on an invoice, your linked cash account (usually a bank account) is credited correctly, and your Trade Debtors account is debited again. This creates a negative balance in Trade Debtors. (Sales Ledger linked accounts)
If you have allocated only a few invoices to your Trade Debtors account, it's simple to correct this problem: Simply reverse or delete the invoices and re-enter them. Make sure that you've specified the correct sales income account before you start to reenter any invoices. (To link an account)
If you've allocated numerous invoices to your Trade Debtors account and your negative Trade Debtors balance is very large, you'll need to use the Record Journal Entry window to correct your account balances. Make a Nominal Journal entry to debit your Trade Debtors account so that it shows the correct balance. Then use the Nominal Journal to credit your sales income account with the amount of the Trade Debtors debit. Continue to record your customer payments using the original, incorrectly allocated invoices, though. As you receive payments, MYOB Accounting Plus will continue to debit your Trade Debtors account correctly and will also correctly increase the balance in your linked cash account.