Is it possible not to display the form for submitting a message to the advertiser for a certain post category? And if so, how can it be done?

<?php if(osc_item_category_id() != 1234) { ?>
 .... contact form button ...
<?php } ?>

I had duplicated this code but it doesn't work. Like this  <?php if(!osc_category_id()==1234)  { ?> ---...-----<?php } ?>       also, but that doesn't work either

The code you suggested does not hide the button in any category

I belive you've not kept 1234 there.

Of course, I have put my own id

Why you used osc_category_id instead of propsed osc_item_category_id function?

I used what you suggested
<?php if(osc_item_category_id()!=231)  { ?>

but also this
<?php if(!osc_item_category_id()==231)  { ?>

to no avail.

If you use
<?php if(osc_item_category_id()!=231)  { ?>
  ...something...
<?php } ?>

and put some code here
and item category id is 231
then this code will not be visible on that item.

I put exactly what you suggest
<?php if(osc_item_category_id()!=231)  { ?>
  ...something...
<?php } ?>
 I want it to be invisible but it doesn't work. Unfortunately, the button is visible in all categories

This is the full code that I use to make the button invisible in category 231 but it doesn't work. The button is visible:

<?php if(osc_item_category_id()!=231)  { ?>
          <a href="<?php echo bet_fancy_url('contact'); ?>" class="open-form contact btn mbBg" data-type="contact"><i class="fa fa-envelope-o"></i> <?php _e('Apply and send CV', 'beta'); ?></a>     
        <?php } ?>

Then osc_item_category_id() != 231

You suggest the same code again I tried it and it doesn't work

try intval(osc_item_category_id()) != 231

If that does not work, you must go to PHP team and report them that 1 != 1 does not work anymore.

No, it didn't work. If there is another way, let's continue the effort for a solution, otherwise we give up the effort.