Note: The following setting is only accessible to the Workspace Owner. Regular workspace admins won't be able to access these settings.
Settings & administration
In your Slack app, select your workspace name in the top left corner. Under Settings & administration, select Workspace settings from the drop-down list.
Go to the Permissions tab at the top.
Scroll down to Channel Management and choose Expand.
For People who can create private channels, change the setting to Everyone, except Multi-Channel Guests.
For People who can create public channels, change the setting to Everyone, except guests.
Save your settings.
App Management Settings
From your desktop, click your workspace name in the top left.
Select Settings & administration from the menu, then click Manage apps to open the Slack App Directory.
Click App Management Settings in the left column.
Turn OFF both Require App Approval and Only allow apps from the Slack App Directory.