@foodlfg Thats right, thats what i did and i got the output telling me i need java15. but installing java15 did not solve it unfortunately.
I was thinking the environment variable should be different two so i tried changing it to /usr/lib/jvm/java-15-openjdk-amd64 but thats probably wrong.
EDIT: just got it to work. had to use this command:
export INSTALL4J_JAVA_HOME=/usr/lib/jvm/java-15-oracle