Get orbital period of exoplanet from light curve using astropy.timeseries
The second light curve you show has no obvious periodic behaviour and I cannot see any sign of a planetary transit. The period-finding algorithm appears to be working correctly.

The planet (if it exists) is supposed to be one of the smallest planetary candidates found by Kepler and will have a barely detectable transit (depth of order 0.004%). The small light curve variations you see are either from the star or remaining instrumental artefacts. If you wish to verify or otherwise the results published by Morton et al. (2016), who show no light curves, you will have to do considerably more work to "detrend" the other variability in the light curve.