Your generation will fail if you have any previous periods that have not been accepted and generated. You'll need to check all previous periods to see which one has not been generated. To do this, follow these steps.
Click Rostering, then click Generation.
Click Visits, then select the relevant branch.
Click the bin icon on the failed week.
Click Delete, then the status will show as submitted.
The status will then show deleting this will take a second to delete.
Click Add new, then click Save.
The status will show as submitted, then generating and finally generated.