Users being unable to register with participant registration phase enabled

When users try to register, they get an error message saying that the configuration is not set up correctly (see image 1).
However, I have enabled the participant registration phase.
What could cause the problem?

You have to define the different options and prices of the registration form before you can enable the registration for participation.
Please go to the page:
Overview => Settings
... and scroll down to the section “Settings for the Participant Management of ConfTool”.

The best way to define the settings for the participant registration form is to start from the top and work all the way through to the bottom, keeping to the order of the available options (see image 2).

Please note that the items directly visible of the participant registration form are defined on the pages:

• “Manage Event/Item Groups” is used to create the different sections of the form.
• “Manage Events and Items” is used to define the actual articles / events on the form.

You can find more detailed information here:
Configuration of the Participant Registration Module