Hey there! Just to confirm, are you adding a thumbnail on a video you're uploading? If so, learn more about it here: https://yt.be/help/STpv . Otherwise, please share more specifics so we can look into it.
Hmm, that's strange, we'd like to request for more details you can share about the issue you're experiencing to be sent in a feedback report using these steps: https://yt.be/help/feedback . This is to help the appropriate team further investigate.