Downloading ELF Process failed of Project

You should have another error message before this one that tells you why the download failed.

Do you have a system id component in your sopc project? If not then add it first. It will ensure that you are using the correct configuration. IF you don't have a license for the Nios CPU you are using, then there is an opencore evaluation window that will open after you configure the FPGA. Don't close this window or the CPU will stop working.

Check also that the design is properly constrained and meet all timing requirements.