Template parse errors: 'mat-icon' is not a known element

In Angular 9 or greater you should

import { MatIconModule } from '@angular/material/icon'

and then

imports: [
    // other imports...
    MatIconModule, // <-- here
],

The MatIconModule is missing from your imports.

imports: [
    BrowserModule,
    FormsModule,
    HttpModule,
    MatButtonModule,
    MatIconModule, // <-- here
],