<p>As <span class="mention">@wajeed-msft</span> notes, this is happening because your administrator turned off access to external applications.</p>
<p>Assuming “Allow sideloading of external apps” is also turned off you’ll get the list on the <a href="https://learn.microsoft.com/en-us/MicrosoftTeams/enable-features-office-365?ui=en-US&rs=en-US&ad=US#apps">page you found above</a>. If you scroll through that list, you will see “Incoming webhook” listed.</p>
<p><a href="https://i.sstatic.net/vFMG6.png"></a></p>