NOTE: check the settings in the Configuration ... Stock ...

These should be the settings you need:
Check stock level true
Subtract stock true
Allow Checkout false
Then decide if you want the Product to disappear from the Catalog when the stock quantity gets to <= 0 ...
Products status in Catalog when out of stock should be set to
Show Products when out of stock

0= set product status to OFF
1= leave product status ON
Set to 0, the Product is set to status off, and will not show ...

Set to 1, the Product will show, but show as Sold out ...