You need to include them in a group that has the "can log in" global permission.
I suspect you've run into the "default settings problem". The default in Confluence (and similarly in JIRA for that matter) is to have a group called confluence-users. This group has "can log in", which is absolutely fine, but then the (bad) design is to use that group generally to allow access to spaces as well, as a default.
This means you have three options
Note that however you do this, you will need to add the user to the "see space" permissions.
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