Favvy will let you know if you’ve selected a layer that isn’t square; this can be resolved by resizing your frame until the width and height are identical (eg. 512x512).
There’s a known issue with the Figma desktop app (only on Windows), which also happens for normal file exports from Figma. When you go to save your file, you may see an “all files” label. If you ignore this and continue by clicking “Save”, it should still save the file with the correct extension and allow you to open it as expected after it has downloaded to your computer. If it still saves the file with a blank extension, you should be able to rename the file to manually append the correct extension to the file name.
Please note, if you’re in China, the accounts server may be blocked by “The Great Firewall of China”. If you’re seeing an activation error, despite using a valid key, you will likely need to use a VPN to resolve the issue.