1. Go to Setup > Site Builder > Set Up Website > Setup tab > Price Level (take not of the configures site price level)
2. View the item record having the error when adding to cart.
3. Navigate to the Pricing tab
4. Add a value on the price level used on the web site. If Base Price is not populated, provide a default value.
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.