User &1 is already using &2

What causes this issue?

&V2& (or a sub-tree of it) is already being processed by user &V1&.

System Response

You cannot makes changes.

How to fix this error?

You can try to process the sub-tree later, or you can ask user &V1& to
finish working with it.
If you do not want to change anything, you can also use the display
transaction.

Error message extract from SAP system. Copyright SAP SE.