There are a few different account types within Xero that each keep track of different pots of money. The liabilities account is for money you owe patients, things like patients buying gift cards would go into this account. Regardless of whether you use the functionality it adds, Flex will expect one to be configured and linked in.
The account will first have to be setup within the Xero portal by either yourself or your accountant (they may have already done this, it would be worth checking with them). Unfortunately it is not something we can do for you however there are plenty of articles online that detail how to add a liabilities account within Xero(https://help.gettimely.com/article/407-using-xero-liabilities
this guide is designed with another piece of software in mind but the start of it is relevant).
Once the liabilities account has been setup within Xero call the support line on 0800 310 2400 and we can take you through linking it into Flex.