SAP system has status 'not modifiable'

What causes this issue?

The system administrator has set the system status to "not modifiable".
You cannot change Repository objects in this system.

System Response

You cannot change Repository objects in this system.

How to fix this error?

Contact the system administrator.
For more information, see the SAP Library under <LS>BC - Basis
Components</> -> <LS>Change and Transport System</>.

Error message extract from SAP system. Copyright SAP SE.