Unable to give a user access to KBA space in Confluence

Yes, I have tried that. When I type the user’s name, nothing appears in the search results. If I enter the user’s email address instead, an Invite button appears, but when I select it, I receive a message stating that the user cannot be invited at this time.

The message then suggests sharing the content by notifying the user. That process appears to complete without any errors; however, the user is still unable to access either the page or the space.

Permissions rather than licensing, and the invite path is a large part of why it looks like licensing.

Start at site level. Settings, then Products, then under Jira Service Management pick Knowledge base permissions, and turn on All logged-in users. Then go per space, in your service space under Project settings > Knowledge base, and set the Who can view dropdown against that linked space to All logged-in users.

That's the one that bites here. The docs are explicit that if "access to both logged-in users and anyone is turned off, project admins can only allow Confluence users to view articles in linked spaces". And Only Confluence users is defined as restricting access "to articles in this space to licensed Confluence users who have access to this space", so the space stays licence-gated whatever your project admin does, and the wording your user is seeing is honest about that.

Worth a look at who holds what as well. Changing the site level setting needs Jira site admin and Confluence cloud admin together, and the docs say that with Jira site admin alone you get read-only access to those permissions, so your admin may well be looking at a screen they can't change.