Google announced that is is temporarily pausing the ability to create custom search style experiments for your related search styles in your Google AdSense account. Google said it hopes to bring this feature back by early July 2021.
This will be paused on May 10, 2021 through sometime in July 2021. Google said “beginning May 10, 2021, you will not be able to create new experiments for your Related search styles in your AdSense account. Your existing Related search experiments that are currently in progress will also stop on May 10, 2021. Please note that this change does not prevent you from creating experiments for other elements of your styles (e.g., search ads, shopping ads).”
Google said its engineering teams are working hard to bring this functionality back by early July 2021.
So when you click into Optimization and then Experiments and try to make a Custom search style, you may run into an issue soon.
Forum discussion at Twitter.