Hi Alessandro, sorry about that, no worries. Let me clarify:
SDM - Service Desk Manager
CMDB - Configuration Management DataBase (part of Service Desk Manager)
CI - Configuration Item
With that, assets can be brought into the MDB via various apps - such as Service Desk Manager/CMDB, or IT Asset Manager. In your case, since that record is designated as an "asset" and not a "ci" - most likely it was brought in by an application other than Service Desk Manager/CMDB - maybe it was brought in by IT Asset Manager or another app. Because of that, the record may be "owened" (as we call it) by that app. If you log into Service Desk Manager, and you go to Search > Configuration Items - and type in the name of that record, when you pull it up in the GUI, try to edit it and set the model there, and see if it allows you to save it. My guess is that it will throw the same error in the GUI.
Hope this helps,
Jon I.