Whilst migrating from Drupal I received the below error:
Fatal error: Class 'EntityPlusControllerExportable' not found in /app/web/core/modules/entity/entity.module
As entity_plus was not enabled, I enabled it via the mysql cli (status=1), just to see if that helped.
Sadly though, this then resulted in the below error:
Call to undefined function entity_metadata_field_text_property_callback()
I am unsure which issue needs to be resolved sadly, but it feels like a migration bug at this point.
Note, this was also posted on the issue queue. Sorry about the cross posting, but in retrospect, this seems like the better place to post it at first until we confirm there is an issue: https://github.com/backdrop-contrib/entity_plus/issues/23