I have installed Eclipse, JDK 14, and Wolfram Workbench on my computer. I have made sure to specify the location of my Mathematica install. However, when I try to open a Mathematica notebook from Eclipse, I am prompted to enter my Mathematica activation key.
However, I don't think this is necessary since my installation of Mathematica is valid. When I exit that window I am greeted with this:
Also note that I specified the Mathematica 12.0 folder in Preferences, not just the Mathematica executable file.