How to fix Xero error "The document date cannot be before the end of year lock date" in Xero

Error code: 

The document date cannot be before the end of year lock date, currently set at dd-mm-yy

Cause:

The user has set a lock date, signifying the closing of the period. Therefore, the transaction, bill, or invoice cannot be edited.

Resolution:

In Xero, try to:

• Change the bill date 
  
  
  
• Remove the lock date
  
  

In Peakflo:

• Go to the Accounting section and under Transactions, find the bill with this error (you can sort by dates to navigate easier). Then, re-sync the bill.