mat-icon is not a known element in angular

I am trying to use an angular material icon for add but I get this error 'mat-icon' is not a known element: 1. If 'mat-icon' is an Angular component, then verify that it is part of this module. 2. If 'mat-icon' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.
the structure of my project is like this I have a modules folder that contains a dashboard folder and I have a folder named shared that it has it owns module. this is my dashboard code

<div class="search-div">
  <button mat-raised-button  (click)="onCreate()">
    <mat-icon>add</mat-icon> Create
  </button>
</div>

this is my app.module.ts

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppRoutingModule } from './app-routing.module';
import { AppComponent } from './app.component';
import { BrowserAnimationsModule } from '@angular/platform-browser/animations';
import { DefaultModule } from './layouts/default/default.module';
import { MatTableModule } from '@angular/material/table';
import { HttpClientModule } from '@angular/common/http';
import {UserInfoService} from './services/user-info.service'
import {MatCardModule} from '@angular/material/card';
import {MatDialogModule} from '@angular/material/dialog';
import { PostsComponent } from './modules/posts/posts.component';
import {  MatFormFieldModule} from '@angular/material/form-field';
import {MatInputModule} from '@angular/material/input';
import { MatIconModule } from '@angular/material/icon';
import { FormsModule, ReactiveFormsModule } from '@angular/forms';

import { NewuserComponent } from './modules/newuser/newuser.component';
import { DashboardComponent } from './modules/dashboard/dashboard.component';

@NgModule({
  declarations: [
    AppComponent,
    NewuserComponent,




  ],
  imports: [
    BrowserModule,
    AppRoutingModule,
    BrowserAnimationsModule,
    DefaultModule,
    MatTableModule,
    HttpClientModule,
    MatCardModule,
    MatDialogModule,
    MatFormFieldModule,
    MatInputModule,
    MatIconModule,
    FormsModule, 
    ReactiveFormsModule
  ],

  providers: [UserInfoService],
  bootstrap: [AppComponent],
  entryComponents:[NewuserComponent]
})
export class AppModule { }

how can i fix that error?