Angular Property does not exist on type 'Object'

Comparing your JSON data and Product interface, you have missed out sprints property in your model.

Via json2ts, your Product interface should be as below:

export interface RootObject {
    id: number;
    title: string;
    description: string;
    accessCode: string;
    createdAt: Date;
    updatedAt: Date;
    sprints: Sprint[];
}

export interface Sprint {
    id: number;
    title: string;
    releaseDate: Date;
    description: string;
    createdAt: Date;
    updatedAt: Date;
    projectId: number;
    specifications: Specification[];
}

export interface Specification {
    id: number;
    title: string;
    description: string;
    duration: number;
    status: number;
    createdAt: Date;
    updatedAt: Date;
    sprintId: number;
}

Another concern is the concern mentioned by @mat, as currentProject data is asynchronous. Either you have to initialize the value to currentProject:

@Input() currentProject: Project = {
  title: '',
  description: '',
  accessCode: '',
  sprints: []
};

Or using Typescript optional chaining (?.) to escape error when currentProject was null or undefined.

@Input() currentProject: Project;

<mat-toolbar>
  <span>{{ currentProject?.title }}</span>
</mat-toolbar>
<div class="data-panel">
  <mat-card>
    <mat-toolbar style="border-radius: 4px 4px 0px 0px;">
      <span>Development</span>
    </mat-toolbar>
    <mat-card-content>
      <span>Access Code: {{ currentProject?.accessCode }}</span>
      <div *ngFor="let sprint of currentProject?.sprints">
        <span>{{ sprint | json }}</span>
      </div>
    </mat-card-content>
  </mat-card>
</div>

Sample Demo on StackBlitz