Thanks for posting in the Community, cleverbeebookkee!
In QuickBooks Online you'll have to select a bank, credit card, or asset account type as the payment account where the funds are taken out to pay for the specific expense. This is the reason why you're unable to select it for an expense transaction. You can use the owner's contribution account for an expense under the Category details section only.

Also, you can check this link to learn more about how to owners contribution and chart of accounts in QuickBooks Online:
Record an owner's contribution or capital investment in your business
Learn about the chart of accounts in QuickBooks Online
Message us back if you need more help with QuickBooks Online. Have a nice day!