Hi there,
It seems you’ve built a custom header.
As the off-canvas menu is a GP feature, the button has to be in GP’s header in order to trigger it.
Can you try using GP’s default site header? Then follow the below steps:
1. You can follow the guide here to center your site logo:
https://docs.generatepress.com/article/centering-logo-navigation/
2. Go to customize > layout > primary navigation, set Navigation Location to `float left.
3. Add this CSS:
.main-navigation ul#menu-primary {
display: none;
}