grant create session, create procedure, alter user 
  to user_admin identified by user_admin;
  
conn user_admin/user_admin

create or replace procedure unlock_user (
  username varchar2
) as
begin

  execute immediate 'alter user ' || 
    sys.dbms_assert.schema_name ( username ) || 
    ' account unlock';
    
end unlock_user;
/

conn chris/chris

grant create session 
  to unlock_app_user identified by unlock_app_user;

grant execute on user_admin.unlock_user to unlock_app_user;

alter user hr account lock;

conn hr/hr

Connection Failed

conn unlock_app_user/unlock_app_user

exec user_admin.unlock_user ( 'MADE_UP_USER' );

ORA-44001: invalid schema

exec user_admin.unlock_user ( 'HR' );

conn hr/hr

select user from dual;

USER   
HR