Why the bookmarker is greyed out

Hi amel181,

We appreciate you reaching out to us.

We’d like to gather additional details that may help us be in a better position to assist you.

1. Are you experiencing the issue on multiple books?
2. Have you had the chance to restart your device? Restart your iPhone - Apple Support

If you answer yes to both questions, please check for a software update and make sure that you are running the latest version of iOS.

While it isn't inherently assured that an update will resolve the issue, updating to the latest software version for your device can help isolate potential software issues. 

Kindly secure a backup and update.

1. How to back up your iPhone, iPad, and iPod touch
2. Update your iPhone, iPad, or iPod touch

Then, use the steps under the "Bookmark a page" section in the article below.

Read books in the Books app on iPhone - Apple Support

Take care!