The currently experimental Web Serial API
your web browser allows RS232 serial port access using a supported
serial port on your local machine. It appears the Web Serial API is
not currently enabled in your web browser (or if it is enabled, then
it may not available in this web browser version).
Try enabling this feature by copying and pasting the related link
shown below into the address bar of your browser and switching the
'Experimental Web Platform features' to 'Enabled'. Note that
currently cutting and pasting this link below rather than clicking
an active link is required for security reasons.