If you are trying to upload a WordPress theme and/or it’s child theme as well, you may come across this error.
You can try a couple things.
Step 1. Deactivate Plugins through CPanel
In the CPanel, go to the File Manager.
Navigate to the /wp-content/ folder.
Inside wp-content folder, you will see a folder called plugins. This is where WordPress stores all plugins installed on your website.
Right click on the plugins folder and select Rename. Change the name of the plugins folder to anything that you like. In our example, we will call it “plugins.deactivate”. Once you do this, all of your plugins will be deactivated.
Retry uploading your theme to see if you get the same error. If so, continue to the next step.
Optional Step 2. Upload Theme to Root Folder
Upload the zip file(s) to the root: home/yoursite name/public_html/wp-content/theme/
Upload there and extract. After that, delete the zip file. Go to wp-admin and continue.