Below are the possible causes, Make sure you verify all below 5 possible causes:

1. Navigate to Teams admin center(https://admin.teams.microsoft.com)-> Click on "Manage apps" under Teams apps -> Click on "Actions" -> Click on "Org-wide app settings" -> Make sure "Let users interact with custom apps in preview" option is enabled

If it is a Azure Bot, Check below settings:

2. Navigate to Azure Bot in https://portal.azure.com -> Click on "Channels" in left navigation -> Make sure "microsoft teams" channel is added

3. [Important] Navigate to Azure Bot in https://portal.azure.com -> Click on "configuration" in left navigation -> Make sure value given in Microsoft App ID is matching with Teams app manifest bot id.

4. If the Bot is Single Tenant bot, make sure app registration is created in same tenant as user is installing the app.

If it is Copilot studio Bot, Check below setting:

5. Navigate to Copilot studio bot -> Navigate to "Channels" tab -> Click on "Teams and Microsoft 365 Copilot" -> Click on "Add channel"