There maybe several reasons for this message, but for most posters here the problem have been, that the server did not accept the AppleID security settings: if your password is insufficient or the answers to the security not at least six characters long, you will get "try again later", without any indication what might be wrong, so check first this document:
Other reasons may be:
- Too much traffic at the App Store, really try again later.
- You may have used an image that cannot be printed - a corrupted or incompatible image file: to check for this, try to use the "Preview Book" button or to print your book to a pdf file. If this also does not succeed, try to find this corrupted image and use a different version.
- Your address information may be inconsistent with Apple's rules: check carefully your zip code, and check the "Language & Text" System Preferences pane: The region in the "Region" tab must be set to the country you are in, and the country must be the country used with your AppleID.
- Your credit card information must be correct and the credit card still valid.
- Check your security settings at My AppleID appleid.apple.com
- The password needs to be acceptable, at least eight characters and a number and an UpperCase letter.
- The answers to all your security questions need to be at least six characters long.
- Your Firewall or antivirus software may prevent the access to the store; you may need to turn them off temporarily.
- Change the security settings at My AppleID, if they are not correct, see Kappy's post here: Re: I forgot my resuce email and my answers for the security questions. I need help please!!
And your book must not have more than 100 pages.