module error in php

Question

recently I have made a form code and attached modules to it. but soon after I have reached an error saying Notice: Undefined index: module in F:\website\webpage\gate.php on line 68

here is the line that was mentioned

   switch ($_GET["module"]) {
    case "settings":
      Include('settings.php');
      break;
    default:
      break;
   }

I am also mentioning the code in the that I showed up ib the link

<a class="nav-link" href="gate.php?module=settings"> Settings </a>

please rectify this error for me and sorry for any in trouble