Exceptions have occurred (see Extras -> Messages)

What causes this issue?

In checking a document, the system has determined that an exceptional
situation has occurred. Typical examples are:
Schedule deviation between current date and scheduled delivery date
of a purchase order item
Validity of cost centers

System Response

For every exception, there is a message. To save you time, the system
does not go to the individual item screens and display the messages, but
collects them and informs you of this.

How to fix this error?

Using the function <LS>Extras</> --> <LS>Messages</>, you can display
the individual exceptions.
A window appears in which the messages for the individual items are
listed.

Error message extract from SAP system. Copyright SAP SE.