A question!
I installed 1.5.7 on my test site and works fine.
Now I want to modify tpl_main_page.php to block some columns.
What do you mean by columns? Are you talking about the sideboxes are entire columns on either side? All of that can be done without editing a file.
In layout settings I have ACTIVATE Responsive Template set to 1.
If you are talking about the Layout Settings under the Configuration menu, there is no such setting for this template.
When I upload tpl_main_page.php from responsive classic template to my own template in my test site the storefront no longer shows up and in my log is the following fatal error..
It could simply be that your OWNTEMPLATE is not responsive. Templates are often not interchangeable.

It seems you are trying to modify a template that is not responsive_classic BUT,
The \includes\templates\responsive_classic\common/html_header is dated 26June20 (that is the file that defines that variable and makes it available for use.)
you list responsive_classic.

We ask for answers to the posting tips for a reason. In this case, a link to your site would really be helpful.

https://docs.zen-cart.com/user/templ...right_columns/
https://docs.zen-cart.com/user/sideboxes/sidebox_list/