Can you confirm that you’ve set the menu to use the “PRIMARY MENU” location?
Also, please note that the Oblique theme does not have a traditional horizontal “header” menu — it uses a “drawer” that pops out when you click/tap the hamburger icons in the upper-left corner of the window, as shown below (I just created this with a fresh WordPress sandbox installation)

If none of this helps, kindly ask for help in the theme’s dedicated support forum, where the developer is more likely to see it and help you out:
https://wordpress.org/support/theme/oblique/
And you really should heed the developer’s advice and stop using this theme, as they’ve clearly abandoned it:
Oblique and Oblique Pro are not scheduled to receive new features going forward.
https://themeisle.com/themes/oblique-pro/
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This reply was modified 6 years, 1 month ago by
George Appiah. Reason: Fixed typo
i think, I have found a solution adding
.sidebar-toggle,
.sidebar-toggle i {
visibility:visible;
}
into Appearence -> Customize -> Additional CSS