I installed kde-plasma on Linux Mint Cinnamon (latest version as of today 12/08/2023), but when I want to apply a global theme I get this error, because I am very new I don’t really understand what it is due to.
It looks like your screenshot captured a window starting to appear, so it’s mostly transparent and its contents are unreadable. I’m assuming this window shows the error message? Can you edit your post to include a new screenshot that clearly shows the error message?
Also note that Linux Mint is not in general known to have great support for Plasma, as its development team’s focus is elsewhere. So when you install Plasma on top of Mint, you’re on your own, and you’re basically create a very DIY experience. For a beginner, I would strongly recommend using a high-quality Linux distro with Plasma pre-installed. A few can be found at Distributions with Plasma and KDE Applications - KDE Community.