Please update my ios .15 update request

If your iPhone, iPad or iPod touch won’t update - "You might be unable to update your iPhone, iPad, or iPod touch wirelessly — or over the air — for one of these reasons", with more detail in this link. --> If your iPhone or iPad won't update - Apple Support

Basically you don't have enough-on device storage for the update. Read the article for solutions. You can either try to delete items, transfer them to iCloud, or try to update using iTunes or Finder.

You need to update your device with iTunes/Finder.