Facebook often update their security settings, and so they should. It's good to have as much control as possible over what you want other people to see.
There are a lot of settings though, and some of them can trip you up if you are assigning Page Roles.
So, if you need to assign a Page Role to a person, and you KNOW they have liked the page, but you can't see them on the list and you are getting an error - try asking them to change these personal settings:
Click your Name >
Click More (next to Timeline, Friends, Photos etc) >
From the drop-down - click Likes >
Click on the Edit symbol - a small pencil on the right >
Click Edit Privacy >
Edited the 'Other' category from 'Friends' to 'Public'.
The person should now appear under People Who Like This Page (in Settings > People and Other Pages).
Another issue that can arise is that the person has their email hidden.
So if they go to their Privacy Shortcuts on the top right, in the blue bar.
Choose 'See More Settings >'
Who can look me up? >
Who can look you up using the email address you provided? >
Make sure this is set to Everyone.
Then use their email address to assign the Page Role.
Once they have been successfully assigned, they can change their setting back to their preferred option.