The Ads broker won’t always have a video ready to play. This can be due to a number of factors, including network issues, frequency of ads already displayed, and proprietary algorythms determined by the ad brokers that they don’t like to disclose. Sometimes it’s simply that the ad hasn’t finished downloading.
My concern is that you’re getting this message instead of the test message. You should always be getting test ads on your project until they are built and ran on machines that are not yours. Always make sure before you run the game to test it that you have the test mode enabled. It’s possible that the system has determined that you’re a developer running on your own machine and denied the ad to protect you from “suspicious behaviour”.
If you’ve ever played a game that lets you get rewards by watching videos, and you get the little popup that no videos are available to watch, this NOT_READY is the message that the program is getting. You won’t always have an ad every time you call for one.