Could not lock all objects for project: &1 is not possible

What causes this issue?

You want to perform action &V1& for the project definition. Since this
action affects the whole project, all subtasks and lower-level phases
and checklists have to be locked. However, at least one task, checklist
item, or approval is already being processed by another user and
therefore it is not possible to lock all objects in the project.

System Response

The system does not perform action &V1&.

How to fix this error?

You can perform action &V1& once all the objects in the project can be
locked.

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.