Cannot display settings field

Modifying form code with echo do_settings_sections(‘rma’) ?
‘something’ : ‘nothing’; renders nothing, so
do_settings_sections(‘rma’) is empty.

With that in mind, I dont see you adding a section in your posted code.
If you add a section to your register_rma_settings() function, and than add this section to your add_settings_field(), than the input will show.

function register_rma_settings() {

    // add_settings_section( $id, $title, $callback, $page );
    // you can use $callback to add some descriptive text otherwise leave it blank, ''
    add_settings_section( 'rma_section', 'some title', 'section_description', 'rma' );

    register_setting('rma_options', 'rma_base_url');

    // we also added our new section id "rma_section" at last argument to the following function
    add_settings_field('rma_base_url', 'Base URL', 'rma_base_url_string', 'rma', 'rma_section');
}

Do you not want to use any section or why you have not created one?

Update
@geoB yeah first I had also the impression that you dont necessarily need to define a section in the add_settings_field() function.
When we read the codex about that function, we see the section argument as optional, like $section (string) (optional)
BUT we can also see that if we did not add a section, the argument will use default.

I also tried inserting specifically 'default' as a section argument, but the setting will not show up. (even not on other settings pages)
I than tried inserting '' as a section argument, but also, nothing is shown.

So it seems that section is not really optional.