Create page username on Facebook - not eligible to create a username

I am creating a page on Facebook. I see a suggestion to create a page username, but when I try to do it, I get an error notification: You're not eligible to create a username.

Why is the creation of page username not working ? How can I create it ?

To create a Facebook page username, you have to match some conditions. The most important requirements to signup for a username are:

• Have at least 25 fans on a page
• Personal account needs to be verified