The cop might have simply lied. But it is more likely that the little drug test kit was crap. I’ve seen articles that say they are simply no good.
If the failure isn’t even reproducible, after the state crime lab’s own results... then they should kick that cop out, at a minimum.
“Tens of thousands of people every year are sent to jail based on the results of a $2 roadside drug test. Widespread evidence shows that these tests routinely produce false positives.”
https://www.propublica.org/article/common-roadside-drug-test-routinely-produces-false-positives