For some reasons my “Manage Pages” menu item was disabled / greyed out. So “Manage Pages” is not working for me in Data Studio for one specific Dashboard (the others seem fine).
Note: Not sure it has to do something with this, the problem occurred after arranging my pages into sections (using the “Manage Pages” function btw 😉 ).

The solution I found is to mess around with the HTML source code:

Right click and click Inspect on “Manage Pages” to jump to the corresponding HTML element.
Right click on “Edit HTML” make sure your are editing the button part.
(Alternative is to open the debugger using F12).

“Manage Pages” is not working

<div _ngcontent-pbp-c336="" class="mat-tooltip-trigger menu-item-container ng-tns-c56-7 ng-star-inserted" style="">
   <button _ngcontent-pbp-c336="" mat-menu-item="" class="mat-focus-indicator mat-menu-item ng-star-inserted" role="menuitem" tabindex="-1" aria-disabled="true" disabled="true">
      <!----><!----><span _ngcontent-pbp-c336="" class="menu-item-text ng-star-inserted">Manage pages</span><!----><!---->
      <div matripple="" class="mat-ripple mat-menu-ripple"></div>
      <!---->
   </button>
   <!----><!---->
</div>

Solution for Greyed Out Manage Pages in Google Data Studio

Remove “disabled=true” and click somewhere else to apply your changes

<div _ngcontent-pbp-c336="" class="mat-tooltip-trigger menu-item-container ng-tns-c56-7 ng-star-inserted" style="">
   <button _ngcontent-pbp-c336="" mat-menu-item="" class="mat-focus-indicator mat-menu-item ng-star-inserted" role="menuitem" tabindex="-1" aria-disabled="true" >
      <!----><!----><span _ngcontent-pbp-c336="" class="menu-item-text ng-star-inserted">Manage pages</span><!----><!---->
      <div matripple="" class="mat-ripple mat-menu-ripple"></div>
      <!---->
   </button>
   <!----><!---->
</div>

Quick Solution

Hit F12 and put this javascript code in the console and hit enter:
$(“span:contains(‘Manage pages’)”).parents(“button”).removeAttr(“disabled”);