Visitor Armor -- How should I upgrade it?

I found this message randomly appearing in OSSS: "装備1個だけを集中して強化するには、全装備を合計15回強化してからできるんだ" It's one of six different messages that appear so I must have missed it. It read "In order to focus your upgrades on a single equipment, you can do so after upgrading all equipments a total of 15 times." I upgraded the suit 29 times total yet I still can't upgrade one equipment twice more than others. The upgrades are also 18/30/12/12 so you can't even upgrade 15 times each if that's what he meant. Any ideas?

Also I got a visitor equip w/ 8 upgrades @ level 16x :f6: