I don’t understand why you’re using flexbox , you should have both the elements inside a single tag
to use flexbox and align them next to each other using row or in a column one below the other and setting the width,
now for you problem you can set margin-left to 400px and the main will align next to the navbar
width: 100%;
max-width: 1920px;
border: 2px solid green;
height: 100%;
-> this margin-left:405px;
}```
[Flexbox MDN check this out it would be helpful in your project if you re planning to use flexbox ](https://developer.mozilla.org/en-US/docs/Learn/CSS/CSS_layout/Flexbox)