https://flatpak.org/setup/Chrome%20OS you can see how this URL is encoded because it has a space between Chrome and OS, it shouldn't even have this space as it is registered as a trademark called ChromeOS ("ChromeOS™ operating system").
for others like https://flatpak.org/setup/Red%20Hat%20Enterprise%20Linux you can put hyphen instead of spaces in the URL.
https://flatpak.org/setup/Chrome%20OS you can see how this URL is encoded because it has a space between Chrome and OS, it shouldn't even have this space as it is registered as a trademark called ChromeOS ("ChromeOS™ operating system").
for others like https://flatpak.org/setup/Red%20Hat%20Enterprise%20Linux you can put hyphen instead of spaces in the URL.