If you have an Agency or Organization account type, all uploaded creative files must be assigned to a particular advertiser, and those files are only able to be used by that particular advertiser. If you would like to use the same creatives for multiple advertisers, you will have to re-upload them again and assign them a new name and new advertiser under the 'My Creatives' tab.
If you have designated the campaign as one that will run election ads, please double check that your creative files are (1) associated with the same advertiser and (2) also labeled as election advertising. Only creatives that were marked as 'election advertising' during the creation process are eligible to be added to a similarly designated 'election advertising' campaign. If they are uploaded incorrectly, you will need to re-upload your creative with the proper designations.