Explain Query Is Not Working in Oracle Db

explain SELECT COUNT(userid) FROM tableone where userid='abc'

That is syntactically incorrect in Oracle. the correct syntax is:

EXPLAIN PLAN FOR sql_statement;

See How to create and display explain plan in Oracle.

For example,

SQL> EXPLAIN PLAN FOR SELECT * FROM EMP;

Explained.

SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    14 |   518 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |   518 |     4   (0)| 00:00:01 |
--------------------------------------------------------------------------

8 rows selected.

Alternatively, you could achieve the same in SQL*Plus:

SQL> set autot on explain
SQL> SELECT empno FROM emp;

     EMPNO
----------
      7369
      7499
      7521
      7566
      7654
      7698
      7782
      7788
      7839
      7844
      7876
      7900
      7902
      7934

14 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    14 |    56 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |    56 |     4   (0)| 00:00:01 |
--------------------------------------------------------------------------