No option to add on Apps showing on sidebar - Only Activity, Chat, Meet and Calendar

Hi vivaaa

Here is Tamer, I'm a community visitor! 

Let me try help you today:

It sounds like you may not have access to the team or channel that you were invited to. In Microsoft Teams, teams and channels are where you hold meetings, have conversations, and work on projects with your colleagues. If you don't see the team or channel that you were invited to in your list of teams and channels, it's possible that you don't have access to it. 

To see if you have access to a team or channel: 

In the desktop or web app, click on the "Teams" tab in the sidebar. 

In the mobile app, tap the "Teams" tab at the bottom of the screen. 

If you see the team or channel that you were invited to in the list of teams, then you have access to it and can join it by clicking or tapping on its name. If you don't see the team or channel in the list, then you may not have access to it. 

If you think you should have access to a team or channel, but don't see it in the list, you can try the following: 

Check with the person who invited you to make sure that you were added to the team or channel. 

Check with your IT administrator to make sure that you have the correct permissions to access the team or channel 

We would recommend you to confirm at the Teams admin center to make sure the permission policies is set and allow all apps: 

In addition, what about you do this: 

At the Teams channel. click the connector and try to add the app 

Please let us know your test results. appreciated it. 

Please also check following facts about Use of Teams apps for external attendees or guest from outside an organization https://learn.microsoft.com/en-us/microsoftteams/non-standard-users 

If my solution was helpful, please mark this question as answered .and consider giving it a thump up. Your positive feedback motivates me to continue assisting others.  Thank you for letting me be a part of your journey to finding a solution! 

Tamer