analogWrite on any PWM pin result in: not capable of PWM Output

Have you updated to the latest version of the Espruino software?
I will try to update from 1v43 to 1v46.

Could you also tell some more on how you wired up the LED?
Ground -> LED -> 100 Ohm -> C6. It lights up with digitalWrite(C6, 1);

Finally, could you try just typing analogWrite()?

analogWrite()
ERROR: Pin UNKNOWN is not capable of PWM Output
Suitable pins are:
A0 A1 A2 A3 A6 A7(AF) A8 A9 A10
A11 B0 B1 B3(AF) B4(AF) B5(AF) B6 B7 B8
B9 B10(AF) B11(AF) B13 B14 B15 C6 C7(AF) C8
C9(AF)
Or pins with DAC output are:
A4 A5

I'm pretty sure it is the software version if it works (and is in the documentation!)