https://www.reddit.com/r/Slack/comments/1tp8rd6/comment/ood49sh/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button
I fix this by going to Preferences in Slack > Notifications > Sounds & Appearance, then I click on "Show an Example". Then I right-click the test notification that appears quickly and go to Notification Settings, where I manually enable Slack.