ORA-00905: missing keyword
there's a space missing before to in 'to myuser'.

BEGIN
FOR X in (select * from all_tables where owner in ('TESTDTA', 'TESTCTL'))
LOOP
    EXECUTE IMMEDIATE 'GRANT SELECT, INSERT, UPDATE, DELETE ON '
        ||X.owner||'.'||X.table_name||
        ' to TESTUSER';
END LOOP;
END;
/