The ComplexData plugin does not exist, after upgrade After my Drupal 8.51 upgrade, I keep getting this error. 


  Uncaught PHP Exception Drupal\Component\Plugin\Exception\PluginNotFoundException: "The "ComplexData" plugin does not exist."


I have Googled it for two days and haven't found any information about what this is or how to fix it. I get the error while using Panels/Page Manager. I did upgrade both modules to the 4/14 release but that didn't help. Any ideas?
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.