Hi Kalvin,
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.
https://community.intuit.com/quickbooks-self-employed-uk
Please do not hesitate to contact us if you require further information and we will gladly assist you with them.
Kindest regards