Hello,

1)  'This promo code is not valid on this website.'

   => check configured 'Website' under section validity : https://tinyurl.com/yfynxe7x 

     it will be applicable for that particular website if it mentioned. Make the field empty if you want to use the same for a multi-website.

2) "sales order doesn't have a sufficient amount of products added to the Sales Order" 

  => could you share the exact warning you get? if the 'minimum amount of purchase' is mentioned in the promotion program then it will be applicable for the amount greater or equal to it.

here you can get basic details about the feature: https://www.youtube.com/watch?v=KSlrxF0KIPY

Thanks!