On piwik 2.16.0 i got this error when i tried to update user country :
PHP Fatal error: Call to protected method Piwik\Plugins\UserCountry\Commands\AttributeHistoricalDataWithLocations::onVisitProcessed() from context '' in plugins/UserCountry/Commands/AttributeHistoricalDataWithLocations.php on line 120
Another user got this error. I just updated my geoip file.
Thank you for the bug report. I issues a fix for this and it should be fixed with the next update of Piwik. Meanwhile you could try to fix it by changing one word in file
plugins/UserCountry/Commands/AttributeHistoricalDataWithLocations.php on your server see https://github.com/piwik/piwik/pull/9915/files#diff-cd85f7dcd7b94984947e0cc41d8dbe19L148 .