Creatives refer to details about the creative/ad itself, such as the ad size, the format, the URL, etc. More info can be found here. A "creative" in Kevel is different from an "ad", as an "ad" refers to creatives that are specifically linked to a flight and thus have targeting and are eligible to serve.
If a creative is set to inactive, then all ads (creatives mapped to a flight) are not able to serve even if they are set to active. Make sure creatives AND ads are active before your campaign goes live.