How to slice letters from string with angular pipe?

Question

When I get Promise error, it consists of a lot of letters so I want to shorten it with a pipe, but error doesn't show. Why?

Service:

 error: any;

async register(email: string, password: string) {
  try {
   await this.afs.signInWithEmailAndPassword(email, password);
  } catch (err:any) {
    this.error = err;
  }
}

TS:

err:any;

register() {

this.auth.register('email', 'password');

this.err = this.auth.error;

}

HTML:

 <mat-error>{{err}}</mat-error>

This code is working but with pipe transform it not working:

@Pipe({name: 'error'});

export class FormErrorPipe implements PipeTransform {

  transform(value: string): any {
    if (value) {
     return value.slice(10)
    }  
    return null;
  }
}

<mat-error>{{err | error}}</mat-error>

Any solution? Thanks!