Trying to loop through XML to extract values in PLSQL

You've already extracted UUID from XML in FROM statement, and in select, figuratively speaking, you now only have four rows

<UUID> n test n </UUID>

So the correct way to extract that data be:

DECLARE
  X XMLTYPE := XMLTYPE(
    '<?xml version="1.0" ?> 
    <StatusUp>
      <G_UUIDs>
        <UUID>1 test 1</UUID>
        <UUID>2 test 2</UUID>
        <UUID>3 test 3 </UUID>
        <UUID>4 test 4 </UUID>
      </G_UUIDs>
    </StatusUp>');
BEGIN
  FOR r IN (SELECT
              EXTRACTVALUE(
                VALUE(p),
                --'StatusUp/G_UUIDs/UUID/text()') AS uuid
                '/UUID/text()') AS uuid
            FROM
              TABLE(
                XMLSEQUENCE(
                  EXTRACT(
                    X,
                    '//StatusUp/G_UUIDs/UUID')
                )
              ) p
           )
  LOOP
    DBMS_OUTPUT.PUT_LINE(r.uuid);
  END LOOP;
END;

If you were to convert this to a sql statement and run it, like so:

WITH sample_data AS (SELECT XMLTYPE('<?xml version="1.0" ?> 
         <StatusUp>
           <G_UUIDs>
               <UUID>1 test 1</UUID>
               <UUID>2 test 2</UUID>
               <UUID>3 test 3 </UUID>
               <UUID>4 test 4 </UUID>
           </G_UUIDs>
        </StatusUp>') x FROM dual)
SELECT p.*,
       EXTRACTVALUE(VALUE(p), 'StatusUp/G_UUIDs/UUID/text()') AS uuid
FROM   sample_data sd,
       TABLE(XMLSEQUENCE(EXTRACT(sd.x, '//StatusUp/G_UUIDs/UUID'))) p;

it would be easy to spot the problem:

COLUMN_VALUE             UUID
------------------------ ----------
<UUID>1 test 1</UUID>
<UUID>2 test 2</UUID>
<UUID>3 test 3 </UUID> 
<UUID>4 test 4 </UUID>

I.e. you're trying to extract the node StatusUp/G_UUIDs/UUID from xml that only contains the node UUID. Instead, if you correct the node you're querying for, you get the right result:

DECLARE
   X XMLTYPE := XMLTYPE('<?xml version="1.0" ?> 
     <StatusUp>
       <G_UUIDs>
           <UUID>1 test 1</UUID>
           <UUID>2 test 2</UUID>
           <UUID>3 test 3 </UUID>
           <UUID>4 test 4 </UUID>
       </G_UUIDs>
    </StatusUp>');
BEGIN
   FOR r IN (SELECT EXTRACTVALUE(VALUE(p), 'UUID/text()') AS uuid
               FROM TABLE(XMLSEQUENCE(EXTRACT(X, '//StatusUp/G_UUIDs/UUID'))) p)
   LOOP
      DBMS_OUTPUT.PUT_LINE('UUID' || r.uuid);
   END LOOP;
END;
/

UUID1 test 1
UUID2 test 2
UUID3 test 3 
UUID4 test 4 

However, EXTRACT and EXTRACTVALUE are deprecated - you should be using XMLTABLE instead:

DECLARE
   X XMLTYPE := XMLTYPE('<?xml version="1.0" ?> 
     <StatusUp>
       <G_UUIDs>
           <UUID>1 test 1</UUID>
           <UUID>2 test 2</UUID>
           <UUID>3 test 3 </UUID>
           <UUID>4 test 4 </UUID>
       </G_UUIDs>
    </StatusUp>');
BEGIN
   FOR r IN (SELECT *
             FROM   XMLTABLE('//StatusUp/G_UUIDs/UUID'
                             PASSING x
                             COLUMNS uuid varchar2(10) PATH '.'))
   LOOP
      DBMS_OUTPUT.PUT_LINE('UUID' || r.uuid);
   END LOOP;
END;
/

UUID1 test 1
UUID2 test 2
UUID3 test 3 
UUID4 test 4 

Equivalent query that you should try running:

WITH sample_data AS (SELECT XMLTYPE('<?xml version="1.0" ?> 
         <StatusUp>
           <G_UUIDs>
               <UUID>1 test 1</UUID>
               <UUID>2 test 2</UUID>
               <UUID>3 test 3 </UUID>
               <UUID>4 test 4 </UUID>
           </G_UUIDs>
        </StatusUp>') x FROM dual)
SELECT *
FROM   sample_data sd,
       XMLTABLE('//StatusUp/G_UUIDs/UUID'
                PASSING sd.x
                COLUMNS uuid varchar2(10) PATH '.');