Thank you for choosing QuickBooks as part of your business, @arif hamid.
There are some features available in QuickBooks that we can or can't perform depending on the current subscription we're using. In your case, recording an expense transaction as a liability to be paid at a later time should be entered as a bill, then bill payment when the payment is made.
The ability to enter bills in QuickBooks Online can only be achieved if you're using the QuickBooks Essentials, Plus, or Advanced plan. That said, it's not available in QuickBooks Simple Start. As a workaround, you may enter an expense transaction just when the liability is paid. Or, upgrade your subscription into a higher version.
To learn more about our QuickBooks Plan and Pricing as well as the features included from each plan, check out the details from this link.
Also, feel free to read the topics from our help articles in case you need related resources while working with QuickBooks in the future.
Post again if you have any other questions. I'm a few clicks away to help. Have a great day and take care always, @arif hamid!