DrJava only works with Java JDK 8 which is available for all major laptop platforms at https://aws.amazon.com/corretto/ (Amazon Corretto) by clicking on the "Amazon Corretto 8" button.  You should uninstall JDK12 (from Oracle?) first.

Sometime within the next year, I hope to release a version of DrJava that works for Java JDK 9+.  Starting with JDK 9, Oracle broke backward compatibility for Java apps that use any of the the "lib" files in the JDK distribution.  Eclipse and IntelliJ have massive staffs funded by paid license revenue or corporate sponsorship, enabling those environments to support Java 9 and its sucessors as soon as they were released..  DrJava is nearly 20 years old.  In the early days, it was partially supported by grants from the National Science Foundation, the State of Texas, and Sun Microsystems and it kept abreast of the latest technology including`support for generic types (as in Java 5) and integrated unit testing (using JUnit) from its inception in 2001.  But now the only support for DrJava is a small fraction of my (donated) time.  For a beginning programmer, the differences between Java 8 and Java 12 are insignificant, but the Java 12 distribution (most importantly library files) is inconsistent with the Java 8 distribution.  Amazon still supports Java 8 because many applications like DrJava still rely on that environment.  In the progression from Java 1 to Java 8, each new version of Java augmented the previous one but supported a high level of backward compatibility.  (I have not tried it, but I think all of the older versions of DrJava will run on Java JDK 8.  I am not aware of any incompatibilities.)  In contrast, none of the released versions of DrJava released since 2001 will run on Java JDK 9, 10, 11, 12, 13, 14, ...

Roll back to Java JDK 8 and enjoy ...

Best,

Corky