add a logo to the bootstrap navbar code example

Example 1: bootstrap 4 navbar with logo and nav on right

Example 2: bootstrap navbar logo center

Try this:

.navbar {
    position: relative;
}
.brand {
    position: absolute;
    left: 50%;
    margin-left: -50px !important;  /* 50% of your logo width */
    display: block;
}
Centering your logo by 50% and minus half of your logo width so that it won't have problem when zooming in and out.