Hello, if I enter a banner html code in the advertisement area and then press the save button, the html code disappears again, why is that?

In whichever file the banner areas are located, I would like to insert the HTML code into each individual area.

you mean into themes banner? which theme?
as far as I can remember, this usually happens due to some security layer on hosting that does not allow especially javascript code like adsense (but often also endup with 403 error).

This can be tested by entering simple string... if this is saved, it's security.

Thanks for answering. What is the simple string?

Yes, when I insert letters, numbers, symbols and even spaces it is displayed on the website. But an HTML code is not saved.

And if I insert the code in file area the advertisements are also visible on the website.

File area?
Which area you mean?

item.php and main.php if I insert the banner html code here into both it works

Where can I find the code for the right and left side banner?

Probably header.php in theme folder

Paste the HTML code in footer.php for left, line 262 for right, line 269