Yahoo Gemini functionality must be enabled at the Business entity that contains your Advertisers.
First, navigate to Admin > Businesses:
Click the down arrow next to the Business you wish to modify, and select Edit:
Under "Advertising Platforms", click the toggle to enable the Yahoo functionality. These settings are cascaded down to the Advertisers which live within the Business.
To double check if Yahoo has been correctly enabled for Advertisers in that Business, navigate to Admin > Advertisers and look for the Yahoo icon next to your Advertiser.