If you encounter the message "Warning: Sections may be missing some billable items. Please check the invoice layout has included allocations for all billable item types" while creating an invoice, it means that your invoice layout might not be configured to include all necessary billable items.
To resolve this, navigate to LEAP Menu > Settings > Application Settings > Accounting > Layout under Invoice, and ensure that your invoice layout includes all relevant billable item types, such as payment items.