VCs showing "This File Could Not Be Found" after moving folders and combining catalogs?

Hey! I'm using Lightroom Classic.

I copied all of the photo folders from a hard drive to a portable SSD, then combined three different lightroom catalogs into one master catalog. After I combined them, a number of photos say, "This File Could Not Be Found." Then have an exclamation mark in the top right corner despite being on my SSD. On settings, it says the location of the photos are the SSD. How do I "find" the files? 

Thank you in advance!