Diagnosis
The last delta load is still running; no new requests possible.
A repeat request is currently running for this DataSource.
You can only request a new delta once this loading process iscompleted.

Procedure
Check the status of this delta request in the Monitor.
If the load has been terminated for some reason, remove the error;
Set the request to red or green using QM action, so that the next deltaor repeat can be requested.