Java Development Kit (JDK).
To use Java command-line tools you need to install a Java Development Kit (JDK). You might get this, or a similar message if your software (or app) requires JDK, but it is not installed on your machine. For instance if you are using Eclipse.
Which version of Java JDK to use depends on your app. You can always reinstall if it does not work the first time. To download the Java JDK you must register with oracle.com.
From the list of the different JDK versions available for download, please chose one that fits your needs. This is easier said than done, and might require a trial and error approach The safest bet is to download the Long Term Support (LTS) version offered.
Go to the Oracle Java JDK download page (version 17 in this link), and select the JDK version required by your operating system (Windows, Linux, macOS). Accept the Licence agreement and download. Follow the installation instructions in the downloaded file.