Why Did You Receive the Error?
This error occurs because your RosterElf Payroll Cycle is set to fortnightly but your Xero account's Payroll Cycle is set to weekly.
A warning message as per below will indicate this is the issue.
How Do You Fix this Issue?
You will need to log into your Xero account and review all the employee files under the "Employment" tab to ensure they have a "Payroll Cycle" set to fortnightly as per your RosterElf settings. Alternatively, if you need to have a WEEKLY payroll cycle, you will need to update your RosterElf account to weekly which you can read about how to do HERE.