Cross-system locks (CSOL) exist (see long text)

What causes this issue?

Cross-system object locks (CSOLs) exist for some of the objects to be
included in the request, which means that the object lists of the
requests cannot be merged.

System Response

The action was terminated.

How to fix this error?

Attempt to include the object lists of the request individually ("Object
List of Request"). When including individual object lists, a dialog box
occurs if lock conflicts exist. This dialog box may allow you to ignore
the conflicts.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.