Solution 1:
This error occurs because WordPress only supports uploading ZIP-compressed plugins. Other file formats are not accepted.
To fix this:
This should resolve the issue.
Solution 2:
A simple alternative is to manually install the theme via FTP:
1. Download the theme package and unzip it on your computer.
2. Upload the entire theme folder to:
/wp-content/themes/
3. Go to your WordPress Admin Dashboard, navigate to Appearance → Themes, and activate the newly uploaded theme.
This bypasses the WordPress uploader and works even if ZIP uploads fail.
Solution 3:
Make sure that only the intended files are in the folder you are uploading.
I encountered the same problem and discovered that there was already a ZIP file inside the folder. After uploading that ZIP file directly, the issue was resolved, and the theme could be activated successfully.