The Stock Status is controlled by WooCommerce so make sure that you have the Stock Quantity set high enough that it won’t trigger the product to be changed to Out of Stock. You can edit both fields using the bulk editor.
Ensure that the “Stock Quantity” value for the parent Variable Product is higher than zero. Also make sure that the “Manage Stock” value is set to “no” for the Variable Product and the Variations.
Another option is to turn off stock management under WooCommerce -> Settings -> Products -> Inventory -> Uncheck “Manage Stock”