“Please Enter a Value for Amount” when Submit order in sitebuilder site

Shopper encounters the error “An unexpected error occurred. Please go back and try again. Please Enter a Value for Amount” when submit order a special product checked but special price is not given.

1. Go to Commerce > Websites > Website List
2. Website record: Click Edit
3. Click Setup tab 
4. Take note of the Price Level set
5. Then, search for the Item record that erroring out when adding to cart
6. Click Edit
7. Click Pricing tab
8. Add a value on the Price Level identified on step 4. Or, at least populate the Base Price.

1. Items without price values will not show up on the web store by default.
2. One instance why this error will occur is that the web Price Level might have been changed from the website settings and there is no price value assigned on the new price level or base price on the item record
3. This error also shows up for matrix items. When the child item has no price levels set up, it throws the “Please enter a value for amount.” when adding to cart.