It's a "mistake" many new players make. They rush through the ages and find themselves stuck with lack of resources and strength to fight.
Go slow, be patient. Build up the city instead of pushing ahead. Stockpile resources.
Connect with other players. It's a very social game, and team work is needed. Join a good guild. Get help from friends and guild mates for things you need like goods and blueprints for Great Buildings and not least experienced advice from veteran players. Participate in guild activities like Battleground and Guild Expeditions.
Solve quests to get rewards. Take part in events. Aid all friends, neighbors and guild members. Pick up "incidents".
Many things in this game you can do without rushing ahead. Some players camp in low eras for "ages".
True though, you will likely collect more coins than supplies. No need to make them interchangeable though. When you have played for a short while, you'll have gathered so many of both, that you'll never run short, and it won't matter if you have 10 times as many coins than supplies.