@jeronimo_0152 I just tried, and I was able to save a PNG to my desktop about a 786kb file. They say that maybe it's how your file name is named? I have no spaces, with dashes or underscores. And one period. Don't use multiple periods because it confuses the OS with that . Try that.
If you are using Chrome, try Safari, Edge or Firefox. The way different browsers handle the final download can be a factor. Better yet, open Photoshop Web in your browser's Incognito or Private Mode. This disables extensions and clears the cache, which often resolves conflicts.