Cant assign menu

I am using the Govpress theme and Im trying to assign my menu “Main” to my site but it’s not working, it simply just displays all my pages and items as it would by default

When I go into menu locations, tick “Main Navigation” below the menu and I go to menu locations and assign the navigation to “Main Navigation” it clears out the checkbox and section afterwards and simple acts like it’s not being assigned at all.
It’s like its unable to assign the menu to the location as it just reverts back to not being assigned again.

I’m not sure why this could be happening…
Any ideas?
Thank you.