Bootstrap navbar is not visible

Hello,

I try to integrate the Bootstrap navbar with the class ‘navbar-collapse’ into my WP theme (a theme that I built from scratch), and it is not visible. I checked in the code inspector and I saw there is a class .collapse, coming from boostrap.min.css which is doing this.
.collapse {
display: none;
}
When I uncheck this class the nav becomes visible.
I don’t want to modify the bootstrap.css structure by eliminating the .collapse class. I tried to set up an !important rule in my custom css to make the nav visible, I even tried an online rule but nothing works. That class seems to go above all. It might be something related to WP, but as I am quite new in wordpress theming I really don’t know what I can do.
I’ve never faced this problem before, anyone can help me please?
Thanks