Slack app

Again, Slack can be accessed via a web browser, so you can most likely continue accessing your Slack channels via your web browser without needing to upgrade your macOS version.

Then again, keeping current with your operating system and security updates is a good idea, so upgrading macOS is a good idea.

There are thousands of users in the company I work for using the Slack app, and most of them have the default Macintosh HD volume name.

Try a different web browser for you download.

Try the Slack version in the Apple Store. https://apps.apple.com/us/app/slack-for-desktop/id803453959?mt=12

Slack can be used from your web browser, and I prefer it that way mainly because I have have multiple tabs open to Slack, so I can Walk-And-Chew-Gum while working in Slack. I can switch browser tabs so I do not need to loose context when I'm in a DM session with someone, while doing Slack searches, while reading unread posts in other channels.

I found the Slack app too limiting. Plus it tended to eat more CPU and memory than open tabs in Safari.

Just my opinion.

Best ask Slack Support. It’s their app.

As vast numbers of Macs use Macintosh HD as the name of the local device , there’s something odd about however Slack is (not) working here.

According to the following Slack webpage, the app is also available in the Mac App Store (with a link there):

https://slack.com/downloads/mac

Using the Slack app, you can Right-Click (Control-Click) on a Channel in the Slack app and get the URL that you can use in your web browser.

When asked if you want to load the page in the Slack app, Cancel, and click on the link that opens the channel in the web browser.

What exactly do you mean... Slack can't accept Macintosh HD ?

Oh, just thinking, are you trying to save it to Macintosh HD instead of your Downloads folder?