get checkbox formgroup from Response API

I have create dynamic checkboxes using this tutorial dynamic checkbox list, now I need to implement dynamic checkbox using an API request, here what I had done,

inquiry-response.ts

interface Item {
  offenceType4: string;
  permSpeed: string;
  actSpeed: string;
  itemAttributes: ItemAttributes;
  offenceLoc: string;
  itemNo: string;
  summonDate: string;
  distCode: string;
  summonType: string;
  hbtlOffndr: string;
  itemAmount: number;
  itemAttributesCount: number;
  summonAmt: string;
  offenceType1: string;
  offenceCode1: string;
  offenceType2: string;
  offenceCode2: string;
  offenceType3: string;
  category: string;
  offenceCode3: string;
  offenceCode4: string;
  respCode: string;
}

interface ItemAttributes {
  attribute5: string;
  attribute4: string;
  attribute7: string;
  attribute6: string;
  attribute1: string;
  attribute3: string;
  attribute2: string;

}
  interface RootObject {
  items: Item[];
  status: Status;
  additionalProperties: AdditionalProperties;
  metadata: Metadata;
}

export class InquiryResponseMultiselectComponent implements OnInit {
  form: FormGroup;
  summons = [];
  data: any[];

  constructor(
    private formBuilder: FormBuilder,
    private modalService: ModalService,
    private inquiryService: InquiryService
  ) {
    this.form = this.formBuilder.group({
      summons: new FormArray([], minSelectedCheckboxes(1)),
    });

    of(this.getSummon()).subscribe(summons => {
      this.summons = summons;
      this.addCheckboxes();
    });
  }

  ngOnInit() {
   this.getSummon();
  }

  getSummon() {
    this.inquiryService.getData().pipe(map((item: RootObject) => item.items))
      .subscribe(item => {
        this.data = item;
      });
  }

  private addCheckboxes() {
    this.summons.map(i => {
      const control = new FormControl();
      (this.form.controls.summons as FormArray).push(control);
    });
  }

I got error state that Type 'void' is not assignable to type 'any[]' when I want to stimulate an async request by using the RxJS operator of(). also got problem when on the private addCheckboxes(). need advise and help how to solve the problems.