I have tried to install iOS 12 multiple times but it keeps saying that there is an error. What can I do to successfully install the update?

See If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support. If you're trying to update using iTunes try over the air instead, or vice versa.

tt2

I had this same problem using my MB Pro. The problem turned out to be that I was using a generic USB to lightning cable. When I tried it using my Apple cable, it worked.