Rex: Another Island

You can view your coin progress in the top right corner of the screen, alternatively you can press the zoom out button to see in which areas you still need coins.

Although the achievement description says "Find all 777 coins", collecting all 777 coins isn't enough. You also have to find the door that leads to the secret 777 ending. This door is located right above the Forest teleporter. Teleport to Forest, you see 2 birds moving up and down to the right of you. Jump on the first bird and double jump to the platform to the right without hitting the second bird. Now jump to the left on the second bird and double jump up to the tree. On top of the tree you notice that rain is falling on some hidden blocks, proceed to jump up these hidden blocks and you'll find the 777 ending door there.

Although the achievement description says "Find all 777 coins", collecting all 777 coins isn't enough. You also have to find the door that leads to the secret 777 ending. This door is located right above the Forest teleporter. You can't easily reach it from that teleporter's location, though. You have to go through Caves first. Alternatively, teleport to the Empty Town, and walk all the way to the left until you reach the place where the cave reaches the surface. From there, go left and climb the tree by jumping on a bird. You'll find the 777 ending door there.