Thank you for reaching out to TubeBuddy. A recent update to the YouTube UI for the Video Details page is causing a bug that is preventing the Thumbnail Generator from loading. We are working to resolve this now. In the meantime you can still access the Thumbnail Generator by going to the TB Menu at the top of your Studio Dashboard (or Video Details) page and and scroll through the list of most popular tools (pictured below) and it is available here.
In some circumstances we are seeing an issue where the Thumbnail Generator is not loading regardless of this workaround. Our developers are aware and they are working towards a resolution, though we do not at this time have an ETA for full-diagnosis and fix.