By default, the metric unique visitors is visible only when the period is set to view the reports for a single day. Example:

For other periods like week or month this metric won’t be visible. To make this metric visible, you need to update your config.ini.php and set below configurations.

enable_processing_unique_visitors_multiple_sites = 1

enable_fingerprinting_across_websites = 1

Note: Setting enable_fingerprinting_across_websites = 1 decreases your user’s privacy.

If you find this interesting, see also how do I enable “Unique Visitors” metric for yearly reports and custom date ranges?