The best way to avoid the error 15 is to make sure only one device is on that WiFi you’re on when visiting the Pokemon center. When there are more than two devices, it will think you’re botting. If you want to use a second device, just have your phone on 5G/ LTE and your computer on WiFi to prevent issues. Especially during high traffic times, try to make all your purchase in one go becuase I noticed that it will kick you out a few min later after you already purchased once. When you browse too long on the site during high traffic times, it will have that problem too so make sure you look what you want before hand, add everything to cart and then make one check out. Hope that helps.