OK, that's a good start with the details. The more detail, the easier it is for us to help you. What's your phone and iOS version?
You mentioned you have iCloud Photos enabled. Is it enabled on both your phone and MBP? If so, the sync happens via iCloud over wi-fi, and you don't need to connect your phone to your MBP to do it manually.
This Apple support article describes the steps to sync via iCloud or sync manually by connecting your phone to your Mac and use the Finder. From the basic info you provided, it almost sounds like you're trying to do both at the same time, and that won't work.
This is strange because iPhoto should not run on Catalina. Do you use iPhoto in a virtual machine running a former macOS? Or is this a typo and the app you are running on your Macbook Pro is another one, i.e. Photos?
It is important to understand which app exactly you have your pics in on your Mac.