This is most likely caused by the Discount analysis being of the wrong type. You can check the Default Codes assigned as follows:
Click System, Currencies
Locate the currency code that the transaction was originally posted as (example GBP) and click the View Record, eye icon.
On the Trading Currency record window, click the Defaults tab. Under the Sales/Purchase heading will be all the default Analysis Codes for this Trading Currency.
Locate the Analysis Code from the error and make a note of the Analysis Code type this falls under:
Sales/Purchase Code (Invoices, Credit Notes, Adjustments)
Sales Bank Code (Payments or Receipts)
Sales Discount Code (Remittance or Bulk Receipt where a discount is applied)
The next step is to check the Analysis Code within the Debtors ledger to make sure that the Analysis Code Type is the correct type according to the Sales Defaults from the Trading Currencies window. To do this, click Debtors/Credtiors, Settings, Analysis
Locate the Analysis and click the View Record, eye icon.
On the Details tab, you will then see the Type this Analysis Code has been designated as.
It will not be possible to change the analysis type while the code is set as a default in the currency record, and so it may be simpler to create new records of the correct type and then update the Currency Defaults.