ERROR TypeError: Cannot read property 'invalid' of undefined angular 4

You Just forgot about Template variable

Add this to your input #bankname="ngModel" and it will work with you

<input ngModel name="bankname" type="text" #bankname="ngModel" class="form-control" placeholder="Name" required> <br/>

<div class="alert alert-danger" *ngIf="bankname.invalid">bankname is required</div>

Your control reference in the view is bk_name. Replace bankname with bk_name.

<div *ngIf="bk_name.invalid && (bk_name.dirty || bk_name.touched)" class="alert alert-danger">
<div *ngIf="bk_name.errors.required">Name is required.</div>