This is something of an understatement. :-)

When solving problems you need to examine the evidence with care, and report your observations with extreme care.
I don’t seen any reference to a thing called “free_space_block” in my article – so how am I supposed to interpret your question ?
I will take a guess (based partly on your later comment that there is a 1MB free extent at the end of the file) that you have some object(s) in the user_recyclebin or dba_recyclebin that you have not yet purged.

Show us the last few lines of the report for each file in the tablespace, and I may produce another guess.
You might also tell us which version of Oracle, and check that the block size is 8KB.

I tried the above steps , I have my last free_space_block marked as 151672 and my dba_extents gives the maximum database block as 120448 .Now as you suggested I tried to reclaim (151672 -120448)* 8 which comes to around 243 MB – which from the total 1184 MB comes to about 942 MB . So I tried the following :-

alter database datafile 8 resize 942M

I get error :- ORA-03297: file contains used data beyond requested RESIZE value

So I made it

alter database datafile 8 resize 980M

Still I get the same error .So for testing purposes I made it

alter database datafile 8 resize 1184M same error .

I am confused since dba_extents is not showing anything beyond 120448 block number and dba_free_extents is showing free blocks till 151672 .What could be the reason ?

You might want to check the recyclebin.
Objects which have been dropped but kept in the recyclebin appear in dba_free_space as free.

I’ve added an update to the article to point this out.

One question – I have some blocks that are marked as ‘free’ by your script but rebuilding Index or moving tables into the tablespace does not use them. I have 50% free in the tablespace but cannot move segments into these free blocks so cannot reduce the datafile.

e.g.

11 250889 385544 free free
…
11 385673 412936 free free
…
11 413065 507912 free free

Any idea why?

Moving lob segments is a pain, because they tend to be quite big and moving them is a lot of work.

If they’re in ASSM tablespaces you may get lucky by shrinking them (shrink space), but this can generate a lot of undo and redo, and although it may free up some extents, the ones that are freed may not be at the critical end of the tablespace. For example:

alter table test_lobs  modify lob(text_content) (shrink space);

As far as I know, the only alternative is to move the table and its lob segments at the same time. For example:

alter table test_lobs move 
lob(text_content) store as (tablespace test_8k_assm);

This command will rebuild the table in the current tablespace, and rebuild the lob segment and lobindex segment in tablespace test_8k_assm.

Please advise, and thanks!