Unable to determine a suitable reconciliation key

What causes this issue?

A serious error occurred during automatic assignment of the
reconciliation key. The system was unable to find any open and unlocked
reconciliation keys. The system could not create documents that are to
be created with this reconciliation key.

System Response

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

How to fix this error?

If you have your own implementation for this BADI: Reconciliation key,
check your implementation.
Increase the value for the number of searches by reconciliation keys
(data element PS4S_FIKEY_REPETITION_NO). You define this value during
mass processing for creating FI-CA documents in the field "Mass
processing parameter 1". (Default value for a reconciliation key record
size of 100 is 100).

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.