When manufacturing order is "Marked As Done" - Closed, all stock movements, accounting postings  and product cost are posted in the background.  This is a "Point of no return".  You will never succeed in reopen this order. 

What to do is to use Unbuild (Manufacturing->Operations->Unbuild orders.)  It reverses all postings.  And you will need a new Manufacturing order.

MTO process will always give you issues, when something is not exactly as planned. Like with this issue.  There are three options. The first two, is preffered with MTO process:

1. Cancel the sales order. Make sure all related MO, PO, and Transfers directly created from the sales order, also are cancelled.  Resett sales order to draft, and confirm it again. All orders will be created again, and the process will flow.  This is the quickest way, and convenient if you have small sales order. 

2.  You can cancel the specific sales order line, by changing order qty to 0.  Take care of the outbound delivery also. It is not updated automatic.  Then create a new order line on the sales order.  This will create a new MO.

3. Create and MO without reference to the sales order.  It will deliver to free stock, and in the outbound the delivery you will not get automatic reservation. But it is possible to process picking and deliver anyway.  You can confirm picked qty, without a reservation. This was not possible for versions before V14.

About MTO. Odoo used to strongly recommend MTO. But it can create more hassle than benefits for many businesses. 

I hope this help you.