To address the issue in Odoo Online Standard Version 17, you need to adjust access rights and record rules for the custom group. Ensure that the group has Read, Write, Create, and Delete access rights for both Projects and Tasks. Additionally, modify the record rules to allow users to create and manage tasks within any project they create. This involves setting domain rules for the Project and Task models to include user-specific conditions, ensuring they can see and interact with projects they own or are assigned to. This should enable users to add tasks to their own projects as intended.