Thank you again for raising this! I wanted to circle back and share a related thread where another designer ran into a similar issue — and where ​@Harsh Kharsani was able to identify a couple of key causes. You can check out the reply marked as the best answer here: Publishing website in Figma Sites is unsuccessful. 

Two things to double-check in your file:

1. At the extreme left bottom if there is an icon button with (A?). That means you are using a font that is not a Google font and figma only supports Google font and not machine fonts. 

2. If you are using any Video in the file make sure it is not more than 10,485,760 bytes. 

In my case it was both custom font as well as had a video with more bytes.