header.component.html

  <div class="nav-wrapper">
        <mat-toolbar class="nav">
          <span class="logo">Greenpot</span>
          <img class="logo-img"src="https://www.pinclipart.com/picdir/big/2-23395_clipart-green-leaf-png-clipart-transparent-png.png">

          <span class="example-fill-remaining-space"></span>




          <button mat-button routerLink="sign-in" *ngIf="!isAuthenticated">Sign in</button>
          <button mat-button routerLink="register" *ngIf="!isAuthenticated">Register</button>


          <button mat-icon-button *ngIf="isAuthenticated"><mat-icon>account_circle</mat-icon></button>
          <mat-menu #appMenu="matMenu">

            <button mat-menu-item>Settings</button>
            <button mat-menu-item>Help</button>
            <button mat-menu-item (click)='signOut()'>Log out</button>
          </mat-menu>

          <button *ngIf="isAuthenticated" mat-icon-button [matMenuTriggerFor]="appMenu">
            <mat-icon>more_vert</mat-icon>
          </button>
        </mat-toolbar>

      </div>

header.component.ts

import { Component, OnInit, OnDestroy } from '@angular/core';
import { AuthService } from '../auth.service';

@Component({
  selector: 'app-header',
  templateUrl: './header.component.html',
  styleUrls: ['./header.component.css']
})
export class HeaderComponent implements OnInit, OnDestroy {
constructor(private authservice: AuthService ){
     }

isAuthenticated;


  ngOnInit(){
    this.authservice.getAuthStatus();
    this.authservice.currentAuthStatus.subscribe(authStatus => this.isAuthenticated = authStatus)
  }

signOut(){
  this.authservice.signOut()
}



     ngOnDestroy(){
     }
}

I think you might rename getAuthStatus() at ngOnInit() at header.ts to authStatusListener()