The ComplexData plugin does not exist, after upgrade
I read in another post where an OP wrote that Composer didn't put all the /core files in place, so he manually copied the /core directory and all its files and that solved a similar problem he was facing.

I tried that last night, manually copying the Drupal 8.51 /core folder into my local install. It fixed the problem. I then copied the files into production and all is well. Thanks to all who commented.