Hi, I'm not too familiar with Slack. But I know that many 32-bit-only apps, starting with High Sierra and Catalina, won't work until the app developer upgrades their software to 64-bit: Compatibility of 32-bit apps with Mac OS High Sierra later ithire.com .
This may not be related, but that was my first guess, as you can note in the article: Apple> About This Mac> System Report> Software> Applications (see the ""64-bit version"" column to determine) or better yet, click on ""Legacy Software,"" all 32-bit software is there and probably won't work. If it does, you may need to contact the application developer; if it's not 32-bit, apologies; let me know because something else is going on; I hope that helps!