Some publishers limit the number of people who can read or view an ebook at one time. Unfortunately, eBook Central doesn't allow you to reserve an ebook that's in use, nor will it let you know when the book is free. All you can do at the moment is try again later. If you keep having difficulty accessing the book, contact the library and we can see if additional copies can be purchased.
EBSO FE Subscription Collection ebooks all have unlimited licences, so this should never be an issue on that platform