Drupal 9, How to solve “Mismatched entity and/or field definitions”
It’s very common to find this warning in the status report page of a Drupal site, this happens usually on the development phase while we are adding/removing fields and modules.
In order to solve this issue staring in Drupal 8.7 and above you need to install a contrib module called Devel Entity Updates
As it says in the module doc
This command is a drop-in replacement of the legacy
drush entity-updatescore commands, however
drush updb --entity-updatesis no longer supported. Just run the
drush entupcommand as before.
Just enable the module and run drush entup to fix these fields issues.