When inputting the serial number, we would suggest making sure to put zeros instead of the letter O. Please note as well that serial numbers usually start with "YH". To know your device's exact serial number, navigate to Settings > System > About.
If still no luck, reply here with the serial number of the affected Roku stick for further assistance.
I am having the same issue. The device gets very hot. Regardless of which browser I use, I cannot order the HDMI extender. I am entering the serial number on the back of the stick. I even tried copy-paste from my roku account to make sure I am entering all the right characters. Seems like the HDMI extender is worth a shot if I can only get one. My serial # is YH00A6552101.