BobCAD V21 is quite old and will be a 32 bit application and it is probable that the install you have of Win 10 is a 64 bit one and that could possibly be your problem, only a guess mind you
P.S. I run V21 on a Win XP Pro PC and it runs perfectly, I have tried Win 10 on my other PC for BobCAD V28 and it is a problem with that, Win 10 is still in the "junk" stage from what i can see, it maybe OK for most stuff but for CAD-CAM software not for me, I went back to Win 7 Pro ! ! ! !
Regards
Rob