Document & is blocked by other user

What causes this issue?

You are attempting to access a document which is currently being
processed by another user. This is not possible.

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?

Try again after a while. If the document is blocked for a long period of
time, your system administrator can determine which user has blocked
document &V1&.

Error message extract from SAP system. Copyright SAP SE.