Sometimes the “import theme” feature in Weebly can get buggy and making uploading a theme more difficult than it normal will be. While Weebly is making progress and people are encountering less import issue, but this problem does come up once in a while. This is a temporary issue, and Weebly will eventually work out the problem. In the mean time, here are something you can try to expedite the process.
1) Try using a different browser OR upgrade your current browser. Different browser handle Weebly differently. You may have problem with one, but not the other. So try to use different browser to upload your theme
2) Check your internet connection. When internet connection is not at its best, it can make uploading more difficult. So check to see if you have a slow connection