Quick pointer: If you’re using a current Kali VM, ssh2john is a python script located under /usr/share/john/ssh2john.py
If you have the private SSH key you first need to generate a hash from it that john can work with:
python /usr/share/john/ssh2john.py id_rsa > id_rsa_hash.txt
Run john with rockyou or whatever list you want to use:
john --wordlist=/usr/share/wordlists/rockyou.txt id_rsa_hash.txt