nested formarray within group angular 6

You'll need the following for your scenario:

1. A parent div with formGroupName="schema".
2. Inside that, a div with formArrayName="currencies".
3. Inside that, a div with ngFor="let currencyGroup of currencyFormGroups; let i = index;". Notice that currencyFormGroups is a getter in your Component Class.
4. Inside that, a div with [formGroupName]="i" where i is the index that we created on the fly within *ngFor.
5. Insde that, two inputs with formControlName="percentage" and formControlName="currency" respectively.

.

Here's all these steps translated to code:

import { Component } from '@angular/core';
import { FormGroup, FormControl, FormArray, Validators, FormBuilder } from '@angular/forms';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {

  createPromo: FormGroup;

  constructor(private fb: FormBuilder) { }

  ngOnInit() {
    this.createPromo = this.fb.group({
      'type': ['type'],
      name: ['name', Validators.required],
      description: ['description', Validators.required],
      enabled: ['enabled', Validators.required],
      promotion_type: ['promotion_type', Validators.required],
      start: ['start', Validators.required],
      end: ['end', Validators.required],
      schema: this.fb.group({
        currencies: this.fb.array([
          this.fb.group({
            percentage: 'percentage',
            currency: 'ZAR'
          }),
          this.fb.group({
            percentage: 'percentage',
            currency: 'INR'
          }),
        ])
      }),
    });
  }

  get currencyFormGroups() {
    return (<FormArray>(<FormGroup>this.createPromo.get('schema')).get('currencies')).controls;
  }

}

Template:

<form [formGroup]="createPromo">

  ...

  <div formGroupName="schema">
    <div formArrayName="currencies">
      <div *ngFor="let currencyGroup of currencyFormGroups; let i = index;">
        <div [formGroupName]="i">
          <input 
            type="text" 
            name="percentage"
            formControlName="percentage">
          <input 
            type="text" 
            name="currency"
            formControlName="currency">
        </div>
      </div>
    </div>
  </div>

</form>

Here's a Sample StackBlitz for your ref.

PS: For simplicity's sake, I've considered all the form controls as input. Please make your changes accordingly.