The issue type 'bug' has no estimation field as default, but I want to have an estimation for this. Is the issue type 'bug' defined in the workflows section, or is it not possible to configure predefined attributes?
To add the Estimation field to a project proceed to the Custom Fields settings of the project in question. More information is here: https://www.jetbrains.com/help/youtrack/incloud/Manage-Custom-Fields-Per-Project.html