iPhone 12 my MacBook doesn't not recognize my iPhone
my older MacBook Pro does not recognize my Ihone 12
Did you go through the steps in the related article from which you posted your question?
If your computer doesn't recognize your iPhone, iPad, or iPod - Apple Support
How old is your older MacBook? I have a ten year old MacBook Pro running 10.11.6 and I can backup and sync all of my iOS devices running iOS 14.6.