reset passcode for iphone 12 in high sierra 10.13.6

Hi mathew-77,

welcome to the Apple (user-to-user) Community.

Re: reset passcode for iphone 12 in high sierra 10.13.6 : can't find phone in finder or itunes

Apple support's online article:

Locate and view your iPhone or iPad on your computer - Apple Support

says:

"an icon of your connected device appears in the upper-left corner of the iTunes window.

Click the icon to view details about your device".

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If you still need help:

You might want to check that the connection, the cable itself, and the power point are all working:

Eg: For iOS15 : Connect iPhone and your computer with a cable - Apple Support

and

What to do: If your computer doesn't recognize your iPhone or iPad - Apple Support

More support if needed:

iPhone support website (info. / support / contact):

iPhone - Official Apple Support

All the best :-)