Are you getting an error uploading a thumbnail? You will more likely get the message on Google Chrome since it is popular and gives users the option to install extensions.
As mentioned, you can install extensions and you may have recently installed or updated an extension that has blocking or strong privacy features and now it is blocking you from uploading the thumbnail. Don’t believe me? Sign in with another browser or use the Incognito Mode where the extensions should be off by default. If that works, you now have to hunt and see which extension is the cause or keep on of the many free Internet browsers on hand to switch to for uploading.
Uploading a video to YouTube while trying to upload the thumbnail can cause issues as well so you may want to wait until the video is fully uploaded. Before uploading, be sure that you have one of the many supported browsers for YouTube to reduce or eliminate issues.