If your system is already armed, additional arming commands sent through the website will fail. For example, if your system is currently in Arm Stay mode, a website command to change it to Arm Away mode will fail.
You must first disarm the system through the website, and then send the Arm Away command. Likewise, if the system is in Arm Away mode, a website command to change it to Arm Stay mode will fail.
Rule of thumb: You cannot switch from Arm Stay to Arm Away mode and vice versa through the website. You must always disarm the system first, and then arm it to the desired mode.