It looks like your final binary is not configured in application laucher. Here is how to set your hello.elf in debug configuration settings. In my case it is JLink gdb. But in essence ensure that you configure binary path. In my case I download code directly to DTIM and run. So I use .elf file.
Run -> "Debug Configurations" -> "SiFive GDB SEGGER JLink Debugging" -> "New" -> This Debug configuration is based on Seggar JLink. Here are my Debug Configuration settings: Main: Project: Select your project name C/C++ Application: Your final ELF file