Invoice document still contains messages

What causes this issue?

When checking the Logistics invoice document, the system discovered
errors, warning messages or information messages that have not yet been
displayed on the screen.
The system visualizes the messages that occur using a colored symbol on
the <LS>Messages</> button:
Green: The error log contains information messages
Yellow: The error log contains warning messages, but may also contain
information messages
Red: The error log contains error logs, but may also contain warning or
information messages.

How to fix this error?

For more information on the messages, see the error log.
Select <LS>Messages</>.
You can <LS>hold</> invoice documents that contain errors. If you hold
invoice documents, no update occurs in Financial Accounting and the
system takes no further action, such as creating a letter of complaint
or sending a mail to Purchasing. The document remains in the system
with the status "contains errors" and can be processed further in the
following ways:
Invoice documents that were called up from the <LS>Invoice Overview</>
and held can only be processed further in the <LS>Invoice Overview</>.
Invoice documents that were held in the application <LS>Create Invoice
</> or <LS>Park Invoice</> can be processed further by calling them
from the worklist and from the <LS>Invoice Overview</>.
You can <LS>park</> any invoices containing errors. If error messages
occur in the error log when you save the parked document and these
messages have the origin 'Post', you cannot park the invoice.
If the invoice document is correct and no error messages exist as a
result of the update, result of the update, the system can update the
document, even if there are still warning and information messages in
the error log.

Error message extract from SAP system. Copyright SAP SE.