A fix may be pending -
This was merged to develop Sept 12 but the build errored fix: leave balance reports by Mangesh-Khairnar · Pull Request #18984 · frappe/erpnext · GitHub
This report on Sept 18 confirms the error persists Leave Balance Report Error · Issue #19100 · frappe/erpnext · GitHub
Your error report Server Error: TypeError: object of type ‘NoneType’ has no len() in Employee Leave Balance DocType · Issue #19160 · frappe/erpnext · GitHub
Nonetheless please post your traceback…