This usually happens when the
upload_max_filesize value of your PHP settings is less than the size of the file you are uploading.
If you have an access to the
php.ini file of your hosting, try adding these code in the file:
upload_max_filesize = 64M
post_max_size = 64M
max_execution_time = 300
You can also check http://www.wpbeginner.com/wp-tutorials/how-to-increase-the-maximum-file-upload-size-in-wordpress/
Or you may also get in touch with your hosting company and have then double check the following values above.
Another option would be to upload the theme manually through FTP. Go to wp-content/themes then upload the unzip x folder there. Login to the admin area and you should see X installed in the theme's page.
If you are still not able to upload the theme despite increasing the upload limit to 64MB, please provide us with the admin access to your site in a Secure Note: