I just want to link "Travel" page under "Customers" navigation on the global navigation. I tried using look and feel(Navigation), but I couldn't see under "Customers" site on the global navigation with same site collection.
I'm not sure I completely understand your question. Is "Travel" a page or a subsite of Customers? Are you trying to display it on the global navigation of Customers or of some other site collection?
If you want it displayed on the Customers site you need to make sure that Customers global navigation settings has "Show pages" and/or "Show subsites" selected.
If you want to display it on a different site collection you may have to manually add it as a link.