Hi.

...but nothing inside is displayed.

It displayed as I see.

Also, I would like to move my header over to the center and I have been unsuccessful. It is currently on the right

You can align the menu only with CSS and only inside its column.

For center use following code

#sp-menu .sp-column {
    justify-content: center !important;
}
.sp-megamenu-parent {
    margin: 0;
}
.sp-megamenu-parent > li > a, .sp-megamenu-parent > li > span {
    padding: 0 15px !important;
}

For left use following code

#sp-menu .sp-column {
    justify-content: start !important;
}
.sp-megamenu-parent {
    margin: 0;
}
.sp-megamenu-parent > li > a, .sp-megamenu-parent > li > span {
    padding: 0 15px !important;
}