As a Slack workspace administrator, you may have a setting turned on that requires all apps be pre-approved before a user can install it to your workspace. In order to approve the requested app, follow these steps.
When a user discovers a Slack app that they would like to install (i.e twine for Slack), they will find the app listing in the Slack app directory.
When they go to add the app, they will either be permitted to add the app to your workspace, or they will receive a prompt to submit a request to the App Manager if pre-approval for apps is required.
When they submit a request, the App Manager and Workspace administrator receives a DM from Slackbot and an email.
Once you approve it, the user gets a notice that the app has been approved and they can now install the approved app.
As an administrator, you can view all of your installed apps, requests, deactivated apps and your app management settings by clicking on the down arrow next to your Workspace name, clicking "Settings & administration" and then "Manage apps".
Once you're in the App Management Settings page, you can review you user app requests, your app settings and turn off the pre-approval setting under "App Management Settings".