This is my example data:

CREATE TABLE xml_test (
    xml_col XMLTYPE NOT NULL);
/* */
INSERT INTO xml_test VALUES (XMLTYPE('<ns1:SomeRootNode xmlns="http://www.exampleurl.com" xmlns:ns1="http://www.someotherurl.com">
  <ns1:SomeParentNode>
    <ns1:SomeRepeatingNode>
      <ns1:SomeDataNode>12345</ns1:SomeDataNode>
    </ns1:SomeRepeatingNode>
    <ns1:SomeRepeatingNode>
      <ns1:SomeDataNode>23456</ns1:SomeDataNode>
    </ns1:SomeRepeatingNode>
    <ns1:SomeRepeatingNode>
      <ns1:SomeDataNode>12345</ns1:SomeDataNode>
    </ns1:SomeRepeatingNode>
    <ns1:SomeRepeatingNode>
      <ns1:SomeDataNode>23456</ns1:SomeDataNode>
    </ns1:SomeRepeatingNode>
    <ns1:SomeRepeatingNode>
      <ns1:SomeDataNode>12345</ns1:SomeDataNode>
    </ns1:SomeRepeatingNode>
  </ns1:SomeParentNode>
</ns1:SomeRootNode>'));
COMMIT;

This works perfectly fine:

SELECT b.*
FROM   xml_test a,
       XMLTABLE('//*:SomeDataNode'
           PASSING(a.xml_col)
           COLUMNS 
               data_value NUMBER(6) PATH '.') b

|DATA_VALUE|
|----------|
|12345 |
|23456 |
|12345 |
|23456 |
|12345 |

But this throws an ORA-00979 error:

SELECT data_value,
       count(*)
FROM (
 SELECT b.*
 FROM   xml_test a,
        XMLTABLE('//*:SomeDataNode'
            PASSING(a.xml_col)
            COLUMNS 
                data_value NUMBER(6) PATH '.') b)
GROUP BY data_value

I can't see anything obviously wrong here (but I have been looking at it for a while so may be missing something obvious). Any ideas why I'm getting an error?

select banner_full from v$version;

BANNER_FULL
-----------------------------------------------------------------------------------------------------------
Oracle Database 23ai Free Release 23.0.0.0.0 - Develop, Learn, and Run for FreeVersion 23.9.0.25.07

SELECT data_value,
       count(*)
FROM (
 SELECT b.*
 FROM   xml_test a,
        XMLTABLE('//*:SomeDataNode'
            PASSING(a.xml_col)
            COLUMNS 
                data_value NUMBER(6) PATH '.') b)
GROUP BY data_value;

DATA_VALUE   COUNT(*)
---------- ----------
     12345          3
     23456          2