Hello.
Building instructions are here:
https://h2database.com/html/build.html#buildingUse the jar target.
You need Java 8, 9, 10, 11, or 12. You can set the JAVA_HOME environment variable to a path to installation of appropriate JDK.
Java 13 and later versions can't be currently used by the build system of H2. The generated jar, however, will be compatible with the version on which it was built and with all later versions.