Some things to check for, based on my recent experiences:
Does your video successfully play on your computer? I tried to upload a faulty video to Etsy, and only belatedly realized my video file was an incomplete download from the site I created it on (Canva). I fixed the video, then Etsy accepted it.
Try a different browser, or close and reopen the browser you’re using. I have a Mac, and sometimes Safari gets a little out of whack (as does Etsy!). Sometimes reopening Safari fixes it, sometimes I have to switch to Firefox, and sometimes I have to reboot my computer entirely.
See this Etsy help page for video requirements in terms of size, length, file format, etc.
https://help.etsy.com/hc/en-us/articles/360053206073-How-to-Add-a-Listing-Video?segment=selling
if you’re using a file format not on the list (like a GIF), Etsy may not accept it.
