I don't have the level sifter; so, I have operated the BME280 using 3.3V Arduino -- Arduino DUE. The Serial Monitor is reporting the same error message -- Could not find valid sensor. Check wiring!
As I have played a lot with the sensor and the UNO in terms of changing connections and supplies, it is possible that the sensor has gone out of order in the mean time. The next one (once received), I will check it first using DUE.
Thank you!