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The error claims that the plugin requires that Gradle is run at least with Java 9, but you are running it with Java 8. Can't you run it with Java 9+, especially in a Java 17 project?

Well I am running with J17 already, and the java toolchain is set to that version, too

Well, the error disagrees. 😄 Can you maybe knit an MCVE? Or try running with

--version

to see what it says. Toolchain version should be irrelevant here I think.