IDoc not fully processed

What causes this issue?

The IDoc could not be completely processed as errors occurred in
individual items.

System Response

The items of the IDoc that contained no errors were processed for the
most part; the items containing errors could not be posted as
application documents.

How to fix this error?

Using <DS:TRAN.WPER>error/status tracking</>, determine which errors
occurred and try to correct them. You can repeat <DS:TRAN.WPIA>inbound
processing</> at any time. Only those items that contained errors and
which have been corrected will be re-posted.

Error message extract from SAP system. Copyright SAP SE.