I contacted my hosting support and they said that they are unable to help me with fixing this error and that I should try to get help from a developer or use a different plugin.
But I am just curious, what is included in the “demo data”? Like is it important that I import it or can I just add the “style” on my own? The main thing that I want to add on my website is the header slide and the other category/popular widgets displayed below it. How can I input these?