What causes this issue?

You have selected the document display for a flow which has not been
assigned a document in Financial accounting. This can be for the
following reasons:
The flow has not been posted from Treasury in Financial accounting yet.
A corresponding document must be created at a later point.
In legacy transfer, the flow has been marked as posted because a
corresponding document was already present in Financial accounting. The
document can no longer be assigned to the flow.

How to fix this error?

In the view entitled "Posting information" in the cash flow of the
transaction, the document numbers of Financial accounting are displayed
for the posted flows. Branch to this view and choose a flow there that
has a document number.

Error message extract from SAP system. Copyright SAP SE.