Hello @JoyC50 ,
Sometimes YouTube doesn't communicate the correct thumbnail to us when it comes to video blocks. This can typically be resolved by simply inserting a fresh, new block and the then the URL again. If this doesn't work the next steps would be to:
either re-upload the video to YouTube to generate a new link for it, then try plugging that new link into the video block
or upload a screenshot / image of what the thumbnail is supposed to be, and then insert the video as a linked image block.
... View more