By default users are prevented from deleting assignments, milestones and projects with child records. If your organization has a requirement to allow users to delete assignments, milestones and projects with child records, you can do so by changing the configuration options in the ASM Triggers
Automated Services Manager Triggers. ASM triggers execute certain behavior in PSA. ASM triggers are controlled by the fields in the Triggers custom setting and the configuration options in the ASM configuration group. configuration group A collection of configuration options contained in the configuration groups tab that defines behavior of a PSA object or feature..