<form [formGroup]="form" (ngSubmit)="onSubmit(form.value)" class="form-horizontal">
        <div class="form-group row">
            <label for="inputEmail3" class="col-sm-4 ">Username</label>
            <div class="col-sm-8">
                <input formControlName="email" type="text" class="form-control" id="inputEmail3" placeholder="Email Address" [readonly]="isReadOnly">
            </div>
        </div>
</form>

please try like this change [formControl] to formControlName.

And to set the output to the input field please do the following, point the line this.form.patchValue

import { Component } from '@angular/core';
import { FormGroup, AbstractControl, FormBuilder, Validators } from '@angular/forms';
import { Router, ActivatedRoute } from '@angular/router';
import { EmailValidator, EqualPasswordsValidator } from '../../theme/validators';

@Component({
  selector: 'register',
  templateUrl: './register.html',
})
export class Register {
  public form: FormGroup;
  public email: AbstractControl;
  public username: string;

  constructor(private registerService: RegisterService, fb: FormBuilder, private router: Router, private route: ActivatedRoute) {
    this.form = fb.group({
      'email': ['', Validators.compose([Validators.required])]
      .... etc..
    });

    this.email = this.form.controls['email'];

    this.registerService.getUser({ userId: "asdasd2123da2das" }).subscribe(posts => {
      if (posts) {
          var userObj = posts.json();
          console.log("userObj : ", userObj.data);
          if (userObj.data && userObj.data[0].email) {
            this.email = this.username = userObj.data[0].email;  // ouput : abc@gmail.com
            this.form.patchValue({
                email : this.email
             });

            this.isReadOnly = true;
            this.router.navigateByUrl('/register');
          } else {
            alert("You are Not Autorize to access this Page");
            this.router.navigateByUrl('/login');
          }
        }
    });

I missed the square brackets in formGroup in markup as below

<form formGroup="form">
</form>

and it throws similar error as

ERROR TypeError: Cannot create property 'validator' on string 'form'

Once the square brackets added in formGroup (see below), the error disappears

<form [formGroup]="form">....</form>

For complete information about each kind of form, see Reactive Forms and Template-driven Forms from the https://angular.io/guide/forms-overview website. you are missing actual syntax for Grouping form controls and Registering the control in the template.both are valid in different case

<input type="text" [formControl]="name">
<input type="text" formControlName="firstName">