You are using MAC probably with Safari browser that unzip the file by default.
Do the following so your zip files don’t open automatically after you download them:
1) Go into Safari Preferences.
2) Page titled General.
3) Scroll down to the bottom.
4) Deselect the box with the text “Open safe files after downloading”.
5) The theme will now download as a .zip file which uploads
seamlessly to WordPress.
Or use another broswer/PC
Or use FTP method