Uniphy Platform Designer Error: "Cannot find sequencer/sequencer.elf"

Correct. I had already done the steps shown in the link you posted. The Terasic article is actually more detailed than the one you posted. Even after carefully going through all of the steps described in the articles, I am still getting the same error as originally mentioned.

I am able to successfully run Nios II Command Shell from the Start Menu (even not as an Administrator). See below. However, I am still getting the "Cannot find sequencer/sequencer.elf" error when attempting to compile my DDR3 Platform Designer project.