Why is the command 'sudo' throwing error "Sorry, user <user_B> is not allowed to execute '/bin/su - <user_A>' as root on <host>

Issue

• While executing *sudo su * command terminal is throwing permission error. Why is system throwing this error?
• Following are the lines put in /etc/sudoers to allow "USER_B" to execute "sudo su - oracle" (and this does not work, what is wrong?)

User_Alias      ORA_ADMINS = USER_B
Cmnd_Alias ORA_CMDS = /usr/bin/sudo oracle su -


ORA_ADMINS      ALL=(ALL) ORA_CMDS

• How to fix this issue?