I signed up for a 365 E5 trial so that I could test Microsoft 365 Defender. Once the trial was setup on MSFT's side, I added each of the security roles(Security Administrator etc) to my user via admin.microsoft.com.
I also assigned an E5 license to the user.
This was yesterday.
Then I am attempting to follow Onboard Windows devices using a local script from 09/01/2023.
Step 1 says:
Open the configuration package .zip file (WindowsDefenderATPOnboardingPackage.zip) that you downloaded from the service onboarding wizard. You can also get the package from Microsoft 365 Defender portal:
In the navigation pane, select Settings > Endpoints > Device management > Onboarding.

In my environment, I assigned the user the Office 365 license and Security Administrator role to the user:
When I singed in Microsoft 365 Defender portal, I am able to see the Endpoint menu:
Try assigning the Office 365 E3/M365 E3 license to the user and check if it resolves the issue as you might have a E5 license without windows and audio conferencing.
When I tried to login with the user with no proper license assigned, Endpoints menu is missing:
As a workaround, you can also sign-in directly to Onboarding link https://security.microsoft.com/securitysettings/endpoints/onboarding or the Endpoints link https://security.microsoft.com/securitysettings/endpoints
References:
Endpoint menu missing in security.microsoft.com - Microsoft Community Hub by Jack_Chen1780
how to enable MDE in security.microsoft.com - missing menu - Microsoft Community Hub by Felix Brand