404 Page

You need to enable insertion on Error 404 page:
https://adinserter.pro/documentation/additional-block-settings#insertion

Button Misc, tab Insertion.

This is where you define insertion position:
https://adinserter.pro/documentation/automatic-insertion

You can use debugging function Show positions to see what positions for automatic insertion are available:
https://adinserter.pro/documentation/debugging

Have you selected Show positions?

Please enable remote debugging and post page address.

I can see the debugging info:

The only available position for automatic insertions seems to be Footer.

Therefore, you can use insertion before HTML element:
https://adinserter.pro/documentation/insertion-before-inside-after-html-element

For example use HTML element .content-wrapper to insert before the “content” part below the menu.

This depends on where you are trying to insert.

To insert before the “content” part below the menu use the HTML element listed above.