Hi Pauline,
Thank you for reaching support. The targetContentNetwork field may only be set to true for campaign channel type MULTI_CHANNEL or campaign channel type DISPLAY with campaign channel sub type DISPLAY_SMART_CAMPAIGN or campaign channel sub type SHOPPING_GOAL_OPTIMIZED_ADS as mentioned in this documentation. This could be the reason you are getting the RequestError.UNSUPPORTED_FIELD_IS_SET error for networkSetting.targetContentNetwork to true when you have specified 'networkSetting' to searchNetwork. If this doesn't address your concern, you could open your concern to the GitHub Issue tracker for the AdWords API as the client library owners are better equipped in assisting you on this matter. Let us know if you have any concern related to Adwords API.
Regards,
Nikisha Patel, Google Ads API Team