Why iOS 12 can’t be installed on iPhone 5s

You said "1.5 GB memory is available" and that could be the reason why you can't update that device.

Read this support guide:

Get help with over-the-air iOS updates - Apple Support

If you still can't update, free some more space on your device, or try updating it using the latest version of iTunes on a computer.

If you don't have a computer, borrow one from your friends or contact a local Apple Store or Authorized Service provider, explain your needs, and they'll help you.

Apple - Find Locations - Location Selector

Regards

Giulio

Try using iTunes to update instead of doing it over the air.

You're quite welcome. Glad you could work it out 🙂.

Giulio