I understand you are having an issue whereby you are attempting to add a new navigation item called “Collections”, however when you add the link and then go to test it, it links to your 404 page, is that correct?
What might be happening here is that the wrong page is being linked somehow. If I could ask you to head to the Navigation page in your Shopify admin.
On the Navigation page, click the title of the menu that you want to edit.
Click Add menu item.
Enter a name for the menu item. This name displays in the menu and can include special characters or emojis.
Click the Link field, and then enter a website address for an external website or select a link type:
Collections - a specific collection or all collections, however here we want to select the specific collection you created earlier including your Scarf, it is not categorized as a Page which is where you might have been going wrong previously.
Can you let me know if this fixes the issue for you, or if it persists? You may also need to remove the previous menu item that leads to the 404 page on your store.
I would suggest doing so to avoid confusion going forward:
Click Delete beside the menu item that you want to remove.
Click Remove to confirm.
Click Save menu.
Once done you should see your new link working in your navigation.
You can read more about navigation and different layouts, tools, and steps on our help center page here.
Can you tell me a bit about your store? It looks like you sell some pretty wonderful apparel, have you had much success for yourself yet, or are you starting out?