... does not implement the delegate 'convertPurchTableFieldToVendInvoiceDelegate'

This is a fun surprise that can take a bit of digging.  When adding fields to the PurchTable table's HeaderToLineUpdate field group, this error can pop up if they aren't accounted for in the convertPurchTableFieldToVendInvoice delegate, which is part of the VendInvoiceTableToLineUpdate class which copies PO fields to the invoice.  

The first step in subscribing is to find the class' delegate in the Designer and copy the event handler method:

Next, paste that method into a new class:

d365StuffClass
{
	[SubscribesTo(classStr(VendInvoiceTableToLineUpdate), staticDelegateStr(VendInvoiceTableToLineUpdate, convertPurchTableFieldToVendInvoiceDelegate))]
	public static void VendInvoiceTableToLineUpdate_convertPurchTableFieldToVendInvoiceDelegate(FieldId _purchTableFieldId, EventHandlerResult _result)
	{
		switch(_purchTableFieldId)
		{
			case fieldNum(PurchTable, DlvMode):
				_result.result(0);
							break;
		}
	}
}

In this example, I'm adding a condition for the DlvMode field.  I don't want it to copy to the invoice, so I'm setting the result to 0.