Cannot add a Project to a Sales Order line - get error : FRM-40212 : Invalid Value for field Project

We cannot add a Project to a Sales Order line - get error : FRM-40212 : Invalid Value for field Project_Number

However, this Project code has been added to previous sales orders.  I have checked the Project set up and it all looks to be correct - the Project is approved, the end date is 2035, the tasks are also valid to this date.  When trying to add the Project code to the sales order line on the "others" tab, the Project is not recognized and we see the error : FRM-40212: Invalid Value for field Project_Number.  This is stopping us from sending invoices to the customer.