You can tap into a puzzle pack to see the purchasing options available for that pack, real money and/or with Coins. However, please note that some promotional and sale packs, such as those with the "SUPER Value" icon, are only purchasable with real money.

You can find Coin packs in the in-game Store in the Sale, Treasure Box, and other seasonal/promotional sections. Also, you can use the in-game Store's search function and search for the term "coins" to find packs available for purchase with Coins. If you locate a pack you like in this method, tap into it and the option for coins should be available for use.
 
If you are still having trouble purchasing packs with Coins, please reach out to us and provide a screenshot of what you're seeing.