I am not so familiar with creating tests for Drupal. Is there a template to follow?
This must be related to https://www.drupal.org/project/drupal/issues/3400483 🐛 LogicException: The database connection is not serializable (for string translations in Ajax callback) [META] Active
Given there is a basic_page content type on your instance, you visit
admin/structure/types/manage/basic_page/fields/add-field
to add a new field.
There, select Entity Reference (or simply Reference) and then choose any option below (Content, Taxonomy, etc). Then click the button to continue.
In the next page, if I select the type (Taxonomy, Content, etc) an error appears on the top. The logs are in my first submission.