I2C sensor not detected on Raspi3
RX and TX are receiving and transmitting lines for UART communication. I2C lines are labeled SDA (Serial DAta) and SCL (Serial CLock).

Your sensor seems to be capable of UART and I2C communication, the first one being the default.
Following the instructions on the site you linked, you switched the sensor to I2C mode, and the meaning of the IO pins changes:

TX -&gt; SDA
RX -&gt; SCL


Your Raspberry Pi has UART as well as I2C on different IO pins.
But you connected the I2C sensor to the UART Port.
 The I2C port is at about the same position in the lower row of the blue adapter board.

For UART communication, connect

Sensor TX &lt;-&gt; RPI RX
Sensor RX &lt;-&gt; RPI TX


And for I2C connect

Sensor TX (SDA) &lt;-&gt; RPI SDA
Sensor RX (SCL) &lt;-&gt; RPI SCL