Hi Urmas,
In general, you are not allowed to edit System Restricted Lookups.
They often have built-in business logic that only allows values to be displayed under certain circumstances or sometimes only for certain object types.
You should not do a direct database write to a System Restricted Lookup since that could create errors if the wrong value is entered in the wrong location.
If you can't successfully make a change in Clarity, xog should be unsuccessful as well by design.
If you want to understand more about how a particular lookup works, you can open a case with support or even ask a specific question here on the communities site.
You can also copy the content of a particular lookup into a new custom lookup and then modify the custom lookup. But you would then have to substitute your own attribute pointing to your custom lookup for the system-restricted one. In addition, any logic that is built into the original look-up would not occur when you choose values from your own custom look-up.
I hope this answers your question. Please let me know if I have been unclear about anything.
Sincerely yours,
Jeanne Gaskill
Senior Support Engineer
CA Clarity PPM