ORA-01036: illegal variable name/number on PL/SQL script

I'm fairly new to the PL/SQL game and I'm attempting to come up with a short script to create logins and grant read-only/read-write privs (making the necessary grants as necessary). However, I am encountering variable scoping issues with it. Can someone please help what I might be doing wrong?

Snipped is as follows:

DECLARE CNT INTEGER; 

BEGIN

  SELECT COUNT(*) 
    INTO CNT 
    FROM dba_users 
   WHERE username = :ParamUserName

  IF (CNT > 0) THEN
    IF (INSTR(:ParamSelectedRole, 'WRITE') = 0) THEN 
      REVOKE UNLIMITED TABLESPACE FROM :ParamUserName; 
      REVOKE READ_WRITE FROM :ParamUserName; 

      GRANT READ_ONLY TO :ParamUserName; 
    ELSE 
      GRANT UNLIMITED TABLESPACE TO :ParamUserName; 
    END IF; 
  ELSE 
    CREATE USER :ParamUserName DEFAULT TABLESPACE USERS TEMPORARY TABLESPACE TEMP IDENTIFIED BY ":ParamUserPassword" PROFILE ELEV_USER; 
    ALTER USER :ParamUserName PASSWORD EXPIRE;

    GRANT :ParamSelectedRole TO :ParamUserName; 
    GRANT CREATE SESSION TO :ParamUserName; 

    IF (INSTR(:ParamSelectedRole, 'WRITE') > 0 ) THEN
      GRANT UNLIMITED TABLESPACE TO :ParamUserName; 
    END IF; 
  END IF;
END;

I'm doing the variable assignments using ODP.NET and parameterizing them using the OracleCommand.Parameters.Add()

All the DDL statements (GRANT, REVOKE, CREATE and ALTER) need to be in an EXECUTE IMMEDIATE such as

EXECUTE IMMEDIATE 'REVOKE UNLIMITED TABLESPACE FROM '||:ParamUserName; 

and

EXECUTE IMMEDIATE 'ALTER USER '||:ParamUserName||' PASSWORD EXPIRE';

Actually, I'd simplify it a bit and put all the external variables in one place at the start.

DECLARE 
   V_CNT INTEGER; 
   V_USER VARCHAR2(30);
   V_ROLE VARCHAR2(30);
BEGIN
  --
  V_USER := :ParamUserName;
  V_ROLE := :ParamSelectedRole;
  V_PWD  := :ParamUserPassword;
  --
  SELECT COUNT(*) 
    INTO V_CNT 
    FROM dba_users 
   WHERE username = v_user

  IF (CNT > 0) THEN
    IF (INSTR(v_role, 'WRITE') = 0) THEN 
      EXECUTE IMMEDIATE 'REVOKE UNLIMITED TABLESPACE FROM '||v_user; 
      EXECUTE IMMEDIATE 'REVOKE READ_WRITE FROM '||v_user; 
      EXECUTE IMMEDIATE 'GRANT READ_ONLY TO '||v_user; 
    ELSE 
      EXECUTE IMMEDIATE 'GRANT UNLIMITED TABLESPACE TO '||v_user;
    END IF; 
  ELSE 
    EXECUTE IMMEDIATE 'CREATE USER '||v_user||
                      'DEFAULT TABLESPACE USERS TEMPORARY TABLESPACE TEMP '||
                      'IDENTIFIED BY '||v_pwd||' PROFILE ELEV_USER'; 
    EXECUTE IMMEDIATE 'ALTER USER '||v_user||' PASSWORD EXPIRE;

    EXECUTE IMMEDIATE 'GRANT '||v_role||' TO '||v_user; 
    EXECUTE IMMEDIATE 'GRANT CREATE SESSION TO '||v_user; 

    IF (INSTR(v_role, 'WRITE') > 0 ) THEN
      EXECUTE IMMEDIATE 'GRANT UNLIMITED TABLESPACE TO '||v_user; 
    END IF; 
  END IF;
END;