Syncing iTunes to iPhone

Some possible reasons:

• An iPhone may only be synced to one iTunes.
• When you say "my new iPhone":
  • Is the iPhone literally "new" - out of a factory sealed box?
  • If not, has it been thoroughly cleansed before you took possession? (It should have been)
• A current version of iTunes is required to communicate to iOS 12.

See also Sync your iPhone, iPad, or iPod using iTunes on your computer - Apple Support.

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