You can solve your issue by set Continue selling when out of stock for your products and I found an answer of Miles- a Shopify staff about how to do it. Here it is:
" For New Products
When creating your product, under inventory, ensure "Track Inventory" is ticked. This is generally ticked by default. Underneath that checkbox, you'll see another option to "Continue selling when out of stock". Once this is ticked, the product will be available for overselling when out of stock.
Bulk Updates for Existing Products
For existing products, you're able to edit them in bulk, so you can make the change once for all existing products. To allow overselling for these products, follow these steps:
By the way, If you have any trouble while you do your business on Shopify, I'd recommend you visit our website: Avada Commerce.
Hope it helps!