Please tell me about “Featured Slider” and demo import

Nice to meet you. I am writing sentences using Google Translate. Earlier, I set this Theme to Wordpress 5.1.1. However, the "Featured Slider" was not displayed. I only select from customization to front page. I came up with the idea of ​​trying to import a demo. However, when I pressed the import button, I got "Error: Internal Server Error (500)". I doubted the Wordpress settings and tried multiple versions with the same results. The same result was obtained with the newly set Wordpress. Do you know the cause?