There is a field called internal_project_id in res_company hold the id of the internal project and another field in res_config_settings related to the field exists in the company so if you delete it this field will be empty and it's a required field  so if you change anything in the settings for any app you will get an error "Internal project"

Solution:

You can go to the timesheet app after enable developer mode then configurations -> Settings -> in the Time off section -> select the new created internal project for project field and save.

BTY: You need to create a task for Time off under the new created project and you can set it the settings of the time off section under the project field.