Several factors can affect the generation of a P45. The P45 will not be available if the final pay period is open or not finalised. Ensure the period which the leave date falls is finalised (even if there is zero pay for the period).

Additionally, errors may arise if there is a payment after the employee's leave date within the same pay period. 

Changes to an employee’s leaving date may necessitate reopening the past pay period to reflect these updates correctly. 

It's also crucial to ensure that details such as tax codes and previous employment figures are entered correctly in the employee's profile settings for P45 generation. 

Furthermore, if encountering an issue where the system does not allow P45 generation, checking that the correct tax year and leave date have been set as per system requirements is advisable. 

Once the final payslip is finalised, The P45 will then be available to generation for the employee.

If the 'Leave Date' field is greyed out, this will be due to there being no open payslips in the tax year. You would need to re-open the employee's last payslip in order for this field to be editable.