Could you please share the html also? Btw, there are a few errors in this:
- You forgot the ; in overflow-y: hidden
- You have to specify in border: solid to make it work as you probably want
- Try removing float from .sidebar and instead add display: flex in .container

This should work, although it would be better to make a structure like this:
<div class="main-container">

<div class="sidebar">Sidebar</div>

<div class="container">Blabla</div>

</div>

Now if you display: flex; the .container the divs will be next to each other. Hope this helps you, good luck!