When trying to cancel sales invoices I get the following errors 
FileNotFoundError: [Errno 2] No such file or directory: ‘/home/upande/frappe-bench/apps/erpnext/erpnext/selling/doctype/pos_closing_voucher’
Hi I created a workflow for sales invoice doctype as a work around for this error, which enables you to cancel the Sales invoice. But was not able to fix the issue to work as normal without the workflow.