Had that problem, too. Jesse found the solution for me in the OpenNtf ODA Slack: Check Designer - Preferences - Plug-In Development - Target Platform. Quoting Jesse: "... entries at least for ${eclipse_home} and for a directory inside your Notes data dir, here C:\Notes\Data\workspace\applications\eclipse. If they're missing, modify an existing platform or create a new one and add an "Installation"-type entry for ${eclipse_home} and a "Directory"-type one for the eclipse directory within your data dir." (https://frostillic.us/blog/posts/058650e080e352178525832b00519d2c)
In my case I was missing the .../notes/data/workspace/applications/eclipse entry which I had to add myself.
