How do I use the coins I have collected from daily challenges?

They just accrue to get you to the badges. They're not used for anything else.

I have this same question.. How do I use the coins earned in the daily challenges?

Same here. I've downloaded a puzzle collection and nothing, took my coins though. It's impossible for me to open the daily challenges.

Coins are just another term they are using to basically describe experience. The coins add to a value that you've collected in a month to put a ranking on your progress either Bronze, Silver, Gold, or Platinum, which achievements are tied to. They are
not used for any other purpose than as a gauge for progress during the months daily challenges.

I have this happening on Solitaire and taptiles as well. very annoying especially as I had completed all daily challenges and have each day green. I was so close. I had achieved Gold coin as well and now it 's showing that I haven't completed any daily challenges
or coins.

Please fix.