Source document comp.code &1/ source type &2/ source key &3 is locked

What causes this issue?

The document is currently being locked by another process.

System Response

The document cannot be accessed.

How to fix this error?

Once the previous process is completed and the document is unlocked, you
can access the document in question.

Procedure for System Administrators

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.