why balance adjustment cannot enter any amount in personal leave accrual

Hi Sallyquestions, 

Thanks for your question about that personal leave adjustment.

This usually happens when the Carry remaining entitlement over to next year isn’t enabled for the Personal Leave Accrual category.

If you need to adjust the balance, use an entitlement with this option enabled, or process an unscheduled $0 pay run and enter the adjustment in the leave hours column.

More information:
https://www.myob.com/au/support/myob-business/payroll/leave-and-entitlements/managing-your-employees-leave/adjusting-leave-entitlements

Regards,
Genreve