I can't copy files from a Windows computer to an iPhone

And a third option would be to use iCloud Drive: Set up iCloud Drive on your Windows computer - Apple Support. You'd copy the files on the PC to iCloud Drive and you'd use the Files app on the iPhone to view them.

I do this frequently, using two different methods, depending on how many files I wish to transfer.

• The easiest thing is this - open email on the PC and
  • Send yourself an email with the files attached.
  • Then open the email on your Apple iPhone and download the files.
• The other method I use - purchase a thumb drive that offers USB and (in your case) lightning ports.
  • Attach the drive to the PC using the USB port. Copy the files to be moved there.
  • Then attach the lightning side to your iPhone and move the files where you want.

I hope this helps.