It's sometimes possible that the 'stock available' cannot be edited (double click doesn't work), and there are 2 possible reasons:
1. 'Track quantity' is not enabled: indeed this option needs to be enable to be able to manage stock on products manually
2.The product uses variants: both 'stock available field and 'track quantity' are greyed out and cannot be modified when a product uses variants. Stock is managed on variants directly - see the article below for more detailed information