Install libappindicator

sudo dnf install libappindicator

Download the Slack rpm from from https://slack.com/downloads/linux

or with wget

wget https://downloads.slack-edge.com/linux_releases/slack-3.0.5-0.1.fc21.x86_64.rpm

Install with

sudo rpm -i ./slack-3.0.5-0.1.fc21.x86_64.rpm