I just experienced this problem, and I also fixed it by reinstalling. But after I reinstalled, I think I realized my original mistake.
I was in the "C/C++" perspective instead of the default "Develop" perspective. The difference is very subtle -- the lack of "FreeRTOS" menu is the main thing.
I think that was my only problem, but just in case, I have attached the .metadata/.log from my old installation. I also confirmed that C:\nxp\MCUXpressoIDE_10.2.1_795\ide\plugins\com.nxp.freertos.gdb.tad_1.0.6.201807131518.jar existed in that installation.
BTW, I ended up in the "C/C++" perspective because I had very little experience with perspectives. A couple months ago, I had gotten stuck in the "Pins" perspective after poking around in ConfigTools, and not knowing how to get back. So I started trying all the perspectives, in alphabetic order. "C/C++" is at the top of the list, and it looked about right, so I just stayed there. I only discovered the FreeRTOS Debug Guide yesterday, so I never even realized I was supposed to have a "FreeRTOS" menu at all.