Hi,
Thanks for posting in!
You can try uploading theme to your server instead of using WordPress upload feature. To upload X theme to your server, please unzip the file and upload the x folder to ../wp-content/themes/ in your server. Please refer to Theme Installation in our knowledge base.
If you still encounter errors, then this could probably be due to file permission errors. WordPress directory especially wp-content folder needs to be 755. Please ask assistance from your hosting provider. This is just a quick fix for them.
Thank you!