Cannot connect database as sys as sysdba using sqlplus

I am new to oracle server 18c (currently taking online course). My problem is sql*plus where I tried to use sqlplus then sys as sysdba (it did not work) and the error appeared below. However, if I use sqlplus / as sysdba (it worked, and I don't know why) I have tried this command: lsnrctl start, but again not working. In addition, I have checked my listener.ora and tnsnames.oraca I have attached my environment set up in .bash_profile Please help me this. Thank you