Why can't a person see my board as an observer when everyone else can?

[Melissa Arter]

I have my board set to public and I've been sharing it with people so they can make a copy. One person can't seem to access it even with the link. She keeps getting this message: 

Let’s add you to this board another way.

Melissa Arter shared this board with you, but to get access, your email address must be added by a Workspace admin.

I don't want to add her to the workspace, I just want her to be able to view the board. Where are we going wrong?

[Gaurav Kataria]

Since multi-board guests are billable (on paid workspaces) only workspace admins can add them. Details here: https://support.atlassian.com/trello/docs/multi-board-guests/