The install button is not visible when the team receives the url in a slack message and they click on the link.
Slack open links in its internal browser view, and from iOS 11 Safary’s sessions and cookies won’t be available. The solution is either copy the link and open it in Safari, or turn of opening links internally in Slack’s settings.
Unable to clear cache on new device
Another alternative: when the page opens inside Slack click the “open in Safari” icon at the bottom right corner to reopen the page in Mobile Safari.