If you are unable to see quantity incrementor on the Checkout page then there could be two possibilities due to which quantity selector is not visible your Checkout page.
Here we will discuss both possibilities one by one to check why Quantity Selector is not visible.
Possibility 1: Hidden using Product Inventory Settings.
If product is set to sold indiviually through product inventory, Quantity Selector will not be visible at the Checkout page.
Step 1: Suppose you have a product “Simple D3 Supplement 6768” in your cart for which the Quantity Selector is not visible.
Edit this product by going to WooCommerce > Products
Step 2: Here click on "Inventory" tab. Untick option labelled as "Enable this to only allow one of this item to be bought in a single order" if ticked.
Step 3: Click on "Update" button to save changes.
Possibility 2 : Hide from product field on Checkout page.
Step 1: Select a Checkout page > Go to the 'Fields' tab and then click on the 'Products' field
Step 2: Click on “Advanced” tab and untick the checkbox labelled “Hide Product Incrementor” to show productor incrementor.
Step 3: Make sure to click on “Update” and hit the “Save changes” button to publish the changes