If you're logging in to the app for the first time, make sure you've allocated the eBook to yourself and opened it in Boost via web browser first. If you're a student, ask your teacher to check the eBook has been allocated to you, then open it in a browser first yourself.
If you've been using your eBook in the app already, check that your allocation hasn't expired, or if you're a student, ask your teacher to check for you. The majority of Boost eBooks expire one year after the date of allocation, with some exceptions expiring after two years.
Please be aware that you will need to download your eBooks in the app before you can use them offline.