Phrases could not be combined (dangerous goods document)

What causes this issue?

You attempted to combine phrases that are found in the dangerous goods
document. However, the corresponding data record(s) in the dangerous
goods document are currently being processed.

System Response

The phrases are not combined.

How to fix this error?

Ensure that the data records are no longer locked by another user.
Start phrase combining again.

Error message extract from SAP system. Copyright SAP SE.