There could be multiple reasons for that issue. However, the common reason is that you might be trying to upload a complete zip file downloaded from Themeforest.
Now, Unzip the downloaded file, and you will get the reddrop-buddies.zip and reddrop-buddies-child.zip file. Only upload the reddrop-buddies.zip file. The issue will be solved.
Note: Here, we have used reddrop-buddies.zip as an example for our Reddrop Buddies – Blood Donation Activism & Campaign WordPress Theme