You can view the error message You must apply for leave separately for each leave year. Submit a new application from 01 Jan 2022/New year while applying for leave sometimes.
This can be due to the fact that your organization's leave calendar year is from Jan to Dec. The application shows the error if an employee's leaves are starting in the one leave year and ending in the next leave year.
To solve this error, you only need to apply for both the leave years separately.