Cannot GET /string/string ( A problem with router adress in Node.js)

Question

I am using express-handlebars for my routers. I have a navbar in my project website and after i get in addpost site and if i press another link, page will get error which is like Cannot Get /blabla/blabla

I realize that, all routes are one word like;/blog/contact etc...

But Add Post route is like /post/new.

And here is my mainrouter.js code

router.get("/contact" , (req,res) => {
    res.render("./site/contact")
})
router.get("/login" , (req,res) => {
    res.render("../views/site/login")
})
router.get("/register" , (req,res) => {
    res.render("./site/register")
})
router.get("/posts/new" , (req,res) => {
    res.render("./site/addpost")
})

and my navbar code,,

<li class="nav-item">
    <a class="nav-link" href="about">About</a>
</li>
<li class="nav-item">
    <a class="nav-link" href="blog">Blog</a>
</li>
<li class="nav-item">
    <a class="nav-link" href="posts/new">Add Post</a>
</li>
<li class="nav-item">
    <a class="nav-link" href="contact">Contact</a>
</li>

edit: I solve the general problem. Now the problem is, i get this error only when i press the add post button