No amounts can be proposed

What causes this issue?

The reference document contains a net posting with cash discount
agreement entered prior to Release 4.0A. In net posted documents, the
cash discount amount is automatically deducted from the amounts entered
for G/L account line items or asset line items.
Prior to Release 4.0A, the cash discount parts were not marked in the
document line items, so it is not possible to make a clear
reconstruction of the amounts entered.

System Response

A warning that no amounts are proposed.

How to fix this error?

You can enter the document, but you will need to fill out the amount
fields again.

Error message extract from SAP system. Copyright SAP SE.