Why can't I buy icons when I have 8458 coins available to spend a checkmark denies my purchase?

I suspect it might mean you do not have sufficient space to place the item that you wish to purchase

Are you talking about the character avatars in the tips shop? Their purchase button should turn from gray to green when you have earned enough gray tip tokens (not gold coins). If that is not happening then you'd have to get help from customer support through the "help" button in the game settings.

BTW, character avatars give no game advantage. Once you buy them, they will be added to the list of avatars that your may choose for your game picture. They will be visible to non-Facebook friends.

I figured it out! You're talking about the gift bushes. Right now, there's a glitch in the automatic space finder, so to place a gift bush, you might have to manually move the bush to an available space. Once you do, the checkmark square will turn green and you can tap to place it. Hopefully, the developers will fix that soon. It is really annoying.

Some players find it helpful to close the bush menu, move to another area of the island with available space, and then select a bush to give. Good luck!