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Ensure that :
There is not already some TwixRiva classes schedulesany Xero classes scheduled. If there is already some TwixRiva class(Xero) classes, delete the reschedule and click on ‘Run post-installation script’ again.
https://login.salesforce.com/lightning/setup/ScheduledJobs/homeThere is not already more than 95 schedules apex classes. Salesforce’s limit on Scheduled Apex is 100. TwixRiva requires 5 slots.