I can't paste a file into system directory
You have run into System Integrity Protection ( SIP ) which protects certain system folders from being changed.

https://support.apple.com/en-us/HT204899

To make the copy anyway, you turn off SIP.  Be sure to turn SIP back on when done. It's a hassle.

https://www.macworld.co.uk/how-to/mac/how-turn-off-mac-os-x-system-integrity-protection-rootless-3638975/

I'd look for an alternative folder to place the file. I'm not a Java person.  I did find:

https://stackoverflow.com/questions/12842063/where-to-install-jar-files-on-os-x-so-other-java-applications-will-find-them

https://aldoblog.com/2002/05/where-to-put-java-extensions-on-mac-os-x/