'mat-form-field' is not a known element - Angular 5 & Material2

Question

You're only exporting it in your NgModule, you need to import it too

@NgModule({
  imports: [
    MatButtonModule,
    MatFormFieldModule,
    MatInputModule,
    MatRippleModule,
 ]
  exports: [
    MatButtonModule,
    MatFormFieldModule,
    MatInputModule,
    MatRippleModule,
  ],
  declarations: [
    SearchComponent,
  ],
})export class MaterialModule {};

better yet

const modules = [
        MatButtonModule,
        MatFormFieldModule,
        MatInputModule,
        MatRippleModule
];

@NgModule({
  imports: [...modules],
  exports: [...modules]
  ,
})export class MaterialModule {};

update

You're declaring component (SearchComponent) depending on Angular Material before all Angular dependency are imported

Like BrowserAnimationsModule

Try moving it to MaterialModule, or before it

You're trying to use the MatFormFieldComponent in SearchComponent but you're not importing the MatFormFieldModule (which exports MatFormFieldComponent); you only export it.

Your MaterialModule needs to import it.

@NgModule({
  imports: [
    MatFormFieldModule,
  ],
  exports: [
    MatButtonModule,
    MatFormFieldModule,
    MatInputModule,
    MatRippleModule,
  ],
  declarations: [
    SearchComponent,
  ],
})
export class MaterialModule { }

When using the 'mat-form-field' MatInputModule needs to be imported also

import { 
    MatToolbarModule, 
    MatButtonModule,
    MatSidenavModule,
    MatIconModule,
    MatListModule ,
    MatStepperModule,
    MatInputModule
} from '@angular/material';

the problem is in the MatInputModule:

exports: [
    MatInputModule
  ]

I had this problem too. It turned out I forgot to include one of the components in app.module.ts

When using MatAutocompleteModule in your angular application, you need to import Input Module also in app.module.ts

Please import below:

import { MatInputModule } from '@angular/material';

@NgModule({
  declarations: [
    SearchComponent
  ],
  exports: [
    CommonModule,
    MatInputModule,
    MatButtonModule,
    MatCardModule,
    MatFormFieldModule,
    MatDialogModule,
  ]
})
export class MaterialModule { }

Also, do not forget to import the MaterialModule in the imports array of AppModule.