The Pi 3 comes with on-board Bluetooth and WiFi interfaces. If these interfaces are not required they can be switched off by editing the boot configuration:
sudo nano /boot/config.txt
Add the following lines to the end of the file:
# wifi off
dtoverlay=pi3-disable-wifi
#bluetooth off
dtoverlay=pi3-disable-bt
Press Ctrl-O to save and Ctrl-X to exit the editor.
The Bluetooth interface service needs to be disabled to stop the error during boot:
sudo systemctl disable hciuart