When I encountered this problem, after doing some research on the net, I determined the problem was most likely caused by the WordPress Maximum File Upload Size, which was set to 8mb by default. The Newspaper theme I wanted to upload was 11 mb.

To increase the default WordPress Maximum file upload size, I added the following lines to .htaccess

php_value upload_max_filesize 64M
php_value post_max_size 64M
php_value max_execution_time 300
php_value max_input_time 300

This resolved the problem. This method was among several possible solutions I found at the following link:

https://www.wpbeginner.com/wp-tutorials/how-to-increase-the-maximum-file-upload-size-in-wordpress/