The button is hidden with no error message when any one of three conditions is not met. Check all three before investigating further.
The three conditions
All of the following must be true for the button to appear:
The product has exactly one option. Products with two or more options are not supported.
The option's name matches the add to cart key you have set in the size recommender settings.
The recommended size's variant is in stock.
When the recommended size is out of stock
If the variant is in stock for most sizes but not the recommended one, the button is hidden for that shopper and the result shows related product suggestions instead.