Linux error.

crit: Microsoft.AspNetCore.Hosting.Diagnostics[6]
      Application startup exception
      System.IO.DirectoryNotFoundException: /root/Themes/

Anyone?

Check if the directory actually exists or if you have suitable permissions for the directory to read/write

Please, try to run the site with Nop.Web.dll located in the directory. If the /root/Themes/ directory exists then check if the user has permission to access the directory & run the process.