I myself haven't uploaded any videos, but a lot of people with the same problem are due to the video encoding, so if you haven't done so, that would be a good idea. The problem could also be your browser/plugins (FireFox, Microsoft Edge, Safari). If you can, try switching to a different one and see if that helped.
I'm not sure if this applies to you, but the red text under the preview says that the thumbnail can't be changed if you've already completed the video upload. If you have done so, you could remove the video and re-upload it. If not, try the two things before.
If all else fails, you can contact customer support (scroll to the bottom of the home page, select "Nico Help", then select "Contact Customer Support"). Sorry I couldn't be much help with this subject! ( 。•_• 。)