Hello,
Please follow the below steps:
1. Go to Pages > Add new or Edit the page.
2. View at the top, you will see the screen options.
3. Click there and Checked on Excerpts.
4. After that Excerpt will visible. Please scroll to down to see it.
5. Please have a look on the Screenshot.
If the problem is not solved, let us know.
Regards,
Acme Supports