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If your "try again later" is minutes or even a few hours, you might just have to wait for the web service to complete the transaction scheduled for that app.

You could use the developer's contact information / support site to see if they are in the middle of a release - the developer knows when the product is released for sale and/or a price change which can cause this error temporarily.

Lastly, you can contact Apple Store support to have them investigate whether it's a problem on their end with the app in question or whether it's a problem with your AppleID / store account. If so, they can tell you if it's on Apple's end to fix or if you have to fix it.

Basically, several things could be broken and you'll need to work through them one by one to reach a solution.