Ran into this issue recently. Here's how I was able to replicate the error "Mismatched entity and/or field definitions",
I unknowingly had an issue with another module that wasn't installed correctly. So, regardless of what module I enabled, I was running into the WSOD, with the exception error.
Drupal\Core\Extension\Exception\UnknownExtensionException: The module we_megamenu does not exist. in Drupal\Core\Extension\ExtensionList->getPathname() (line 519 of core/lib/Drupal/Core/Extension/ExtensionList.php).
I believe when enabling Metatag module, and encountering an interruption like mentioned above, is what causes the install error.
To fix:
- I used composer to uninstall metatag.
- Took care of the error with the above module.
- Cleared cache and reinstalled metatag with composer, and enabled through GUI.
Once I did the following, Metatag was acting as expected, with default configs ready and the error message gone.
Thank you @imclean, I was running into the same issue as op. I can confirm "Provision content" fixed my issue.