Moodle quiz has All-or-Nothing Multiple Choice question type. Unfortunately, this plugin is no longer actively maintained by the Moodle community. In Moodle version 5.0, issues have emerged with using this question type: it is no longer possible to select the correct answer when creating a new question.

Questions created in earlier Moodle versions still work correctly. So if you already have quiz questions using this type and have set the correct answers, you can continue using them. The issue affects only new questions or existing questions you wish to edit.

It appears as though you can select a correct answer, but in practice, the interface does not allow you to do so. And since a question must have at least one correct answer, it cannot be saved.

What to do?

For multiple choice questions, we recommend using the standard Multiple Choice question type, which now includes equivalent functionality to the All-or-Nothing Multiple Choice plugin.