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;
}