Phase cannot be activated due to active packages in other client

What causes this issue?

The phase cannot be activated because other packages in other clients
are also active in the same phase. These active packages cannot run in
parallel.

System Response

The package is not activated.

How to fix this error?

Wait until the other packages in the other clients are completed, or
change the active phase of these packages.

Error message extract from SAP system. Copyright SAP SE.