At a quick glance, the Sweet Ambar Blue SDDM theme has two versions – one for Plasma 5 and another one for Plasma 6. You probably want the one for Plasma 6. You can check which version of Plasma you’re running in System Settings -> About this System -> KDE Plasma Version.
Be extremely careful when installing Plasma/SDDM themes. They are user-submitted, not always reviewed, and can contain arbitrary code. There have been incidents involving malicious damaging code downloaded through Plasma global themes.