Will niacin flush free clean out my system for thc?

NO. Only time will help clear thc. It can take uo to 1 month for thc to clear.
No. It will not remove thc, the metabolite of cannabis, prior to a drug test. It is not a good idea in any case. Niacin makes your face flush / turn red, and someone imagined that this might "flush" toxins from your system. This isn't the place to debate the wisdom of testing for cannabis; if you test with a low level, you might claim it was second-hand smoke. I'm sorry this is happening.