Man I had the same problem, couldn’t open the packs and build new buildings until I claimed the rewards for the missions I did in Mercenaries. After that everything fall into place and started working. You need to push on the button “Claim” to get your rewards, but I didn’t know, the quests were just completed and waiting there which I thought is onather bug 
I thought once you complete the missions the game will automaticaly give me the reward but it didn’t work like that.
Thanks a lot for this post! I had no idea I had to click on the completed tasks to claim the items. Then the “build the Merchant Cart” was still not available. So I exited the village and then entered it again. The Merchant Cart was available, so I built it and after that I could open the packs 
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