This is a common prompt seen among many users and many devices. This article will cover a two ways this can be resolved.
This prompt occurs because the Slack app does not have ample permissions to execute the update installation.
Option 1: Changing the App Permissions
Granting the app read, write, and execute permissions will allow it to download and apply the update. This can be done by running the below command:
chmod 777 /Applications/Slack.app
Steps on how to run scripts via Addigy can be found here:
Creating and Running Scripts on Your Devices (Saved Scripts)
Option 2: Blocking Automatic Updates (Requires MDM)
This option will prevent users from updating Slack themselves, which will block the prompt entirely. This means that new versions of the app must come from Addigy.
This option is accomplished by creating a custom MDM Profile, which can be configured via iMazing. You will want to select and de-select the highlighted key so that the profile passes the proper values in the profile - this is noted by the blue circle next to the key name.
Once you've got your custom profile configured, go ahead and upload it to Addigy using the following instructions:
How To: Configure and Deploy a Custom MDM Profile