System unable to activate any further items

What causes this issue?

The amount that still had to be allocated could not be distributed to
open items. Either all items are active, meaning that no further items
can be activated OR the items that are not yet active have been blocked
and may not be activated.

System Response

You are issued a message to the effect that it is not possible to
distribute the amount according to age.

How to fix this error?

You can write off the amount not yet allocated or generate difference
postings (partial payments, residual items) or deactivate items that
had already been activated.

Error message extract from SAP system. Copyright SAP SE.