Thank you for your query & hope you had a lovely weekend!
With QuickBooks Self Employed account you can only have one account on there so if you have your bank account connected you would not be able to add a Petty Cash account for this the only option would to add or use your cash account for this.
I have enclosed a link to our QuickBooks Community page where you can find step by step guides on how QuickBooks Self Employed works.
Please do not hesitate to contact us if you require further information and we will gladly assist you with them.
QBSE has a Cash account by default... you won't be able to add another cash account but you can add as many bank accounts as you need. Every time you manually create a transaction, it will be added to the Cash account and for petty cash, you will have to manually create a transaction.
Please do not hesitate to contact us if you require further information and we will gladly help you with them.