Hi Pino,
You're probably getting that error because the Campaign that contains the AdGroup of the AdGroupAd has been removed. Once a Campaign has been removed you can no longer mutate any attributes of its AdGroups.
To limit to AdGroups that are neither removed nor are in removed Campaigns, you can do the following:
1. Use CampaignService.get to select all Campaigns with Status = 'REMOVED', and store those Campaign IDs.
2. When selecting AdGroups via AdGroupService.get, include two predicates:
a. Status IN ['ENABLED', 'PAUSED']
b. CampaignId NOT_IN [ list of removed campaign IDs from step 1 ]
Cheers,
Josh, AdWords API Team