Hello there, @Cmckay.

I'm here to assist you in resolving the issue with the missing "Make payment" button in QuickBooks Online.

The options you can see in Action drop-drown will depend on what tab you'll select.  On the Vendors tab, you can see the make payment option, create bill, create expense, and schedule payment. While on the Expenses tab, you can see schedule payment, mark as paid, view/edit, copy, delete, and void. 

Moreover, if you'll select checks, bill payment (check), and expenses, you can only see view/edit, delete or void. Make sure that the type of transaction you'll select is a bill. This way you'll get to see the mark as paid in the dropdown button.

However, if you already clicked "bill" and the mark as paid button doesn't show, we can perform basic troubleshooting.
 

Here' how: 

Let's log in to your account thru a private window. 

Here are the keyboard shortcuts: 

• Safari: Command + Shift + N 
• Google Chrome: CTRL + Shift + N
• Mozilla Firefox or Microsoft Edge: CTRL + Shift + P

Otherwise, let's clear the cache and cookies to fix issues when using QuickBooks Online in your browser.

You might want to learn about tracking your bill in a detailed way. You can read through this article: Record a bill payment by EFT, ATM card, or debit card.

If you require any further assistance with make payment and mark as paid buttons, you can reply to this post. Take care and have a great day!