This is a solution for a problem. I am posting it in the hopes that it will show up in a search for anyone else that encounters the problem.
I wouldn't mind if anyone else posted a different solution.
I must have hit some hot key for a mode change, but I could not figure out how to exit it without deleting the Built-In folder.
The problem was, if I used the Orthogonal Dimensioning tool, I could only measure the width or length of a Closed Polyline. I was completely blocked from measuring any point on a closed Polyline to anything else on the Workspace.
As mentioned above, my only solution was to exit TurboCAD, delete the Built-In folder, re-start and re-load my preferences under Customize.
TC Pro Platinum 2018, 2017, 2016 & 2015 (all with LightWorks & RedSDK) & V21
System: i7-5820K @ 3.30GHz, ASRock X99 Extreme4, 16GB DDR4-2133 RAM, Gigabyte GTX 970, Samsung NVMe SSD 950 (256GB), Windows 7 Pro (64-bit) SP1