The requirements and process for approving your creatives varies slightly for each exchange.
Xandr
To start creative review and approval on Xandr, make sure the creative is active and assigned to an active line item that targets inventory from Xandr.
Creatives must be manually reviewed before they can serve on Xandr. The Xandr creative review team also checks that creatives meet Xandr's Policy for Buying (external site, requires login).
Xandr's review team works 8 AM to 5 PM ET Monday—Friday, and promises a 24-hour turnaround time for creative reviews. Please note this turnaround time can be slightly longer at the beginning of a quarter.
If a creative is rejected by Xandr, contact Xandr support for help. To contact Xandr, please visit https://help.xandr.com/and include your Xandr Creative ID.
Google Ad Manager
A creative must meet Google Ad Manager's advertising policies prior to being able to bid on Ad Manager inventory. In addition to Ad Manager's initial review, creatives may be manually reviewed at any time by Ad Manager while they're serving.