Here's a breakdown of common causes and solutions:

1. Using UntypedFormBuilder without importing ReactiveFormsModule:

   The UntypedFormBuilder service is provided by the ReactiveFormsModule. If you're using reactive forms but haven't imported this module, Angular won't be able to find the UntypedFormBuilder.

   Solution:

   Import ReactiveFormsModule into the relevant module (typically your AppModule or a feature module). For example:

    import { NgModule } from '@angular/core';
    import { BrowserModule } from '@angular/platform-browser';
    import { ReactiveFormsModule } from '@angular/forms';
    
   
    import { AppComponent } from './app.component';
    
   
    @NgModule({
     declarations: [
     AppComponent
     ],
     imports: [
     BrowserModule,
     ReactiveFormsModule // Import ReactiveFormsModule
     ],
     providers: [],
     bootstrap: [AppComponent]
    })
    export class AppModule { }
    

2. Typos or incorrect imports:

   Double-check that you've imported UntypedFormBuilder correctly and that there are no typos in the import statement or when referencing the class.

   Solution:

   Make sure your import statement looks like this:

    import { UntypedFormBuilder } from '@angular/forms';
    

   And that you are using it correctly in your constructor:

    constructor(private fb: UntypedFormBuilder) { }
    

3. Incorrect module scope:

   If you are using UntypedFormBuilder in a lazy-loaded module, ensure that ReactiveFormsModule is imported into that specific lazy-loaded module as well, and not just the root AppModule.

   Solution:

   Import ReactiveFormsModule in the module that declares the component using UntypedFormBuilder.

4. Conflicting Angular versions (unlikely but possible):

   In rare cases, discrepancies in Angular versions between different parts of your project could cause issues. This is less common but worth considering if other solutions don't work.

   Solution:

   Ensure all your Angular dependencies are aligned to the same version. Use npm list @angular/core (and other @angular packages) to check versions. Update dependencies using ng update.