There are (at least) two ways to do this. Either you use JavaScript to hide the field after the page has loaded or you hide it right in the template.
The first approach would look something like:
function handleHiding() {
if( $('#sample_pcris').val() === "Nao" ) {
$(".resultpcr").hide();
} else {
$(".resultpcr").show();
}
}
$(document).ready(function() {
handleHiding();
$('#sample_pcris').change(function(){
handleHiding();
});
});
Second would look like (I am using pretty much random code, but it should give you the idea):
<% if @sample_pcris == 'Nao' %>
<%= f.text_field :resultpcr, class: 'resultpcr', style: 'display: none' %>
<% else %>
<%= f.text_field :resultpcr, class: 'resultpcr' %>
<% end %>
Untested, obviously.