This ain’t a error message but a standard boot message. To properly answer your question we would need to know what board exactly you use, how you connected your parts (what pins, voltage, etc), what parts and of course the full code.
Please remember that the breakout boards for the esp8266 (for example the node mcu) are confusing sometimes. So always double check what pins you want to use and what pins you declared in your code.
For example, if you want to have D0 as the output for something, probably a LED, you will have to declare it as output.
pinMode(16, Output); // set pin to output
Or when wanting to use it as input (for example for sensors) use
pinMode(16, Input); // set pin to input
Why 16 when wanting to use D0 you ask? Take a look at this for more information on how to refer to the pins as input or output . There you can see what pins to use.